I think that casting a spell in-game is one of the actions more difficult to portray for the players. You can at most point your finger at your DM and pretend to scorching ray-him or say Shirak a-la Rastlin when casting light.
What Im asking is if there is some material to add flavor to spells with verbal and somatic components, such as words to be said and descriptions of the gestures to be made.
We are currently playing 5th edition, so material for those spells would be great, but anything at all would be nice too.
Best Answer
It depends on what you mean by 'material'--do novels count? You've quoted Raistlin in your question, which suggests to me you've already dipped into the material I'd suggest--novels set in the various D&D campaign worlds (Dragonlance, Forgotten Realms, etc.) Such novels are full of descriptions of D&D-style spellcasters working their trade. So look at how the author describes their characters' spellcasting. Do they specify the exact words and gestures? What sorts of words do they use to describe spells being cast?
If you don't have any of these novels yourself, and don't have a handy library that you can borrow some from, I believe both Wizards of the Coast and Paizo have some short D&D/Pathfinder (Pathfinder is close enough for your purposes, I believe) fiction available online.
Of course, wizards are hardly unique to D&D settings. If you've got some favorite series that involve spell-slingers of some sort, you can mine them for ideas, too.
A little further afield: Stage Magicians
If you're willing to go out on a bit more of a stretch, why not look at the people who make their livings doing 'magic'? Magic words may not be as popular on today's stage, but stage magicians still use mystic-looking hand-passes to draw the eye while they work. Plenty of their material is available online, so there's another possible resource to draw ideas from.
Foreign Languages As @Kelhendros pointed out, foreign languages can be a great way to come up with verbal components. Translating the name of the spell (or a short phrase related to the spell) into another language can give you your 'magic words'. Thinking about this, you could do some cool things with this--using different languages to imply different spells come from different sources and so forth.
If you're looking for 'official' source materials, I think you're out of luck
I cannot remember a single sourcebook that describes somatic or verbal components in any detail. Perhaps there's one out there, but I'm not aware of any.
One thought here, though--Dragons are closely associated with magic, and the 3.0 Draconomicon had a (very brief) Draconic vocabulary. You could use that for coming up with some magic words.
Some general advice
I like trying to add some flavor to my spell-slingers in-game, but I've found it usually works better to describe rather than act (unless you're LARPing). Just as you rarely swing your arm about as if you were wielding a sword when your fighter attacks, acting your spellcasting is (at least at the tables I've played at) rarely appropriate.
But just as describing your attacks makes swinging a sword more interesting than 'I attack', so too does saying 'I cup a hand while chanting, forming a small orb of lightning that I then hurl at my foes!' make spellcasting more special than 'I cast an energy-substitution (Lightning) fireball'. This is what I've found most effective--rather than trying to act the individual motions and words, use descriptions that have just enough detail for the rest of the group to fill in the details with their imagination. This is where you apply what you've learned from those stories you were reading.
Describe the effects of the spells, too. Saying your fireball 'explodes into a roaring cloud of flame' builds on your spellcasting description. In fact, I find that varying the visual appearance of spells between characters is a great way to give your spellcasters more flavor.
An example
I've cast a lot of Magic Missiles over the years, and they're a perfect choice for this kind of treatment. After all, you can specify just what your missiles look like. I'll compare a couple of my characters who've had very different Magic Missiles.
One character was a little runt of a kobold. His Magic Missiles took the form of little flying kobold heads. :D So when he first cast Magic Missile, I said something along the lines of, 'With a shrill incantation, I thrust my hand forward and a translucent, miniature kobold head shoots forth, flying out to bite the orc for (roll) 4 damage!'
Another character (Pathfinder this time) was a Strix--so a guy with big, black wings. His magic missiles take the form of feathers, so his casting is more like, 'I growl a brief word of power and flap my wings sharply, and a handful of clear feathers shoot out towards my foe.'
Don't overdo it
The trick with this kind of description is to be brief--spells are already going to be taking a lot of 'screen time' just in terms of resolution a lot of times, so you don't want to make it take even longer. Often describing a few iconic spells in detail is plenty.
Additionally, in D&D, you're going to be casting the same spells a lot. So don't describe them every time. If you've gone in-depth once, just mentioning something that reminds everyone else of your former description can be enough--something like 'my feather-shaped Magic Missiles whistle through the air' can help add flavor without taking the same amount of time you spent the first time around. And then, when the evil sorcerer's Magic Missiles are launched with a 'hissed word' and look like flying serpents, you've established just how different the two characters are.