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So far, most "sources" for BA662A chips that have been found online have not panned out. If you're convinced that you really need one (I used one and don't hear any positive difference), your best bet is probably to gut one from a dead or cheap synth that contains one.
Donor Sources
Boss HC-2 Hand Clapper
http://www.sonicstate.com/synth/boss_hc-2.cfm
This little analogue hand clap synth is in a stompbox formfactor. It has a piezo sensor and a trigger input. It makes a clap sound. That's it. If you can find one of these cheap, it's a good candidate for ripping the BA662A out.
Boss PC-2 Percussion Synth
http://www.sonicstate.com/synth/boss_pc-2.cfm
Like the HC-2, this little stompbox has a piezo sensor and trigger input. Unlike the HC-2, it's actually a very cool little synth that makes modular-ish decaying analogue bloops and blips. Rip the VCA chip out of a dead one, and if you must gut a live one, at least replace it with the BA6110 so the synth still works.
Roland TB-303
If you can find one of these flimsy silver plastic synths, by all means gut the chip out of it. In fact, there's a good chance it may have a couple other useful parts inside. This thing is so old, and doesn't sound anything like a real bass player - I can't see any reason not to gut it for parts. Sorry, I can't find a link or photo of this dinosaur, you're on your own here.
Juno? Jupiter? Where else?
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