there are a lot. among the cheap ones i suggest you the big muff. the boss bass overdrive is quite good too.
Boss: ODB-3 bass overdrive Prescription Electronics: Depth Charge etc etc etc There are a lot of them, the best advice I can give is to fine one that let you mix clean sound and with overdriven sound, so your sound don“t loose "body"
The Sans Amp BDDI it one of the best, but I prefer rackmount distortion like the Ampeg SVP-BSP, Sans Amp PSA-1 and the rackmount DI are the best IMHO.
I've heard that the Big Muff is really good for bass distortion,but I've never heard/tested it personally. I've been told that it holds low end really well.
distortion bad. fuzz not as bad. overdrive very good. The boss bass overdrive is not overdrive. it is distortion. find good natural sounding overdrive. ebs and fulltone seem pretty good. sleep deprivation is not pretty good.