Borderlands – Where to get purple rocket launchers

According to the wiki, you need to input specific recipes into the grinder. The recipes, are as follows, and require items of the same rarity in every case but the legendary case:

• Legendary of desired type: 2 legendary weapons + 1 purple weapon of the desired type
• Up one rarity level (specific type): 2 weapons of the desired type + 1 other weapon
• Up one rarity level (any type): 3 weapons of different types
• Up one rarity level (non-weapons) 3 items of same type (grenade mod, shield, etc)
• Assault Rifle: 1 SMG + 1 Sniper Rifle + 1 Shield
• Rocket Launcher: 2 weapons of any type + 1 Grenade Mod
• Shotgun: 1 Pistol + 1 SMG + 1 Shield
• SMG: 1 Assault Rifle + 1 Pistol + 1 Shield
• Sniper Rifle: 1 Assault Rifle + 1 Shotgun + 1 Shield

For all except the legendary recipe, these recipes work for anything up to and including blue inputs, with the potential to output a weapon of the same rarity or greater. If fed moonstones, the grinder will always output an item of increased rarity, and has a chance to add a luneshine effect to the weapon.

Legendaries appear in the vendor at a much higher rate than BL2 - I got three at Level 15 alone. Legendary drops from random enemies are probably as low as they were in BL2...like way low. IF you know what boss drops the one you are looking for AND that boss respawns (many in the TPS do not), then you can farm him/her/it. I wouldn't waste money trying for getting a legendary from the slots -- again, very low odds it they are same as BL2 -- use that money to buy one from a vendor when it appears. It will.