They're on eBay for a few bucks. I bought a NES/SNES bit in the 90's myself from MCM Electronics (had catalogs from freelance VCR repair I did for a resale shop), thought I had lost it but managed to find it on a magnet I used for saving screws. I paid like $5.99 I think.
I did once use it for evil purposes in the days of BlockBuster rentals. Feeling gyped having bought Street Figher II SNES for $60 bucks, and realizing it wasn't even CE or Turbo edition, I rented SFII: The New Challengers, opened them up, swapped out the PCBs, and presto, instant upgrade for the price of a rental... Yeah, twas a dirty thing to do, and BlockBuster never called me on it. Guess it didn't get rented much.