A few years ago I bought a Vader voice changer called Vortex 2 from fynerdyne or hyperdyne, something like that. It was pretty good. They also did a bounty hunter version and I expect that would have been a similar quality. It wasn't cheap though. I'm thinking a couple of hundred pounds at least.
I have the "TrooperTalk" app on my iPhone. I have yet to test it out because my suit is not complete, but it seems to work fine during the tests I've done. I reached out to the guy who made the app, and he told me that a Boba Fett voice profile is in the works.
I have seen TRamp as a popular choice for a few characters. I ended up going with the Micom Boba kit. 54 voice lines. Customizable. Just seemed like the best option to me. There's a YouTube video showing what it does.