Let's combine some of this into a compendium.
Play-by-chat has been at least 80% of my time spent as a GM. It is a fun way to play because it allows immersion and in-character roleplaying to have more depth by allowing players to write their words and actions, which most people are quite comfortable with. But like all other RPG sessions, you need players. So, where to go?
Start with those closest to you.
I have to mention this. Personally, though I have had many excellent roleplaying games with relative strangers, my most consistently good experiences are with current friends and family. Many times I have been surprised to discover that people I know are actually interested in the roleplaying genre. A good number of my former "mere acquaintances" have become good friends this way. I encourage you to casually mention this interest with other people to see if they share it -- you might be pleasantly surprised!
The Internet
Beyond that, we move into the great interwebs -- the realm of cats and anonymous personas. The best part about the internet is its size. Unless you're into Barbie Horse Adventures RPG, you are bound to find like-minded people out there somewhere. (Honestly, you would probably find some players for that, too.)
A great place to start is going to the source, if you decide to use a VT platform. When you make that decision, you should visit their respective forums. All of them have a section for setting up gaming groups. Even if you stick to playing by pure text, your choice of ruleset can provide additional players from the game's official forums and fansites as well. As a Pathfinder DM, you can post to and search through Paizo's Online Campaign registrar.
In addition, there are Epic-Level powers available to you in the form of social media. As ladenedge's answer already helpfully points out, Reddit's LFG section is a great example. In addition, you can search:
Just be aware that when recruiting random people from the internet, you are still vulnerable to some of the side effects that come with PUGs in online games, such as random AFKs, bad attitudes, cheating, and conflict. This can happen with friends in face-to-face gaming, too, but at least you have a relationship basis with those friends already. Be prepared to deal with player problems which are made more complex by anonymity.
It is dangerous to go alone. Here, take this.
Beyond that, have fun, and be a little patient and choosy with forming your group. With the vast population of the internet available to you, I think you can afford to do so.
As you use Roll20 for visuals anyway, I would try to introduce specific text chat tags. You can use them IC or OOC and players can emphasize their need to speak next like:
[very upset, barely can keep her composure]
or how they are willing to not engage
[grabs some crackers from the backpack and watches people argue]
This also helps the other players to understand the position of other players while another player is still speaking.
Best Answer
I've done the online thing. Getting the right blend of hardware and software to make everybody happy (or, more importantly functional) is a task in and of itself.
This isn't a rules situation, so the DM doesn't have any special authority. Unless there's a compelling reason for a particular platform (screen sharing capabilities, hardware compatibility, etc) then your DM is being difficult for no good reason. He should not expect to be treated differently from any other member of the group. If he does think he's special like that, then you may have deeper seated group social contract issues.
If your DM refuses to bow to group consensus of software to use, it's sadly time for a little social engineering. As a group, simply refuse to play these online sessions if he doesn't switch to something that actually works for the group.