Now, original NES controllers are getting fairly expensive these days--particularly the "dog bone" controllers that are round instead of pointy--and the third party reproductions from Tomee et al. are pretty universally reviled for being flimsy, unresponsive and altogether crummy. SNES controllers, on the other hand, are more ergonomic, originals are generally more plentiful and some of the repros (like this one) are actually pretty great. I also already have a bunch of SNES controllers, including an ASCII Pad, which is possibly the finest third party controller ever produced...
So, with all of that in mind, I cobbled together an SNES to NES controller adapter (there are a number of guides online, see also: here and here, among others) using a couple of cheap extension cables, specifically Tomee NES extensions (I found a set of 2 from an eBay seller for $9) and "Gen" 2-pack SNES extensions. I'll post which wires I connected, but it seems the factories that produce these extension cables just pick a handful of randomly colored wires when they put them together, so don't assume they'll apply to your own cables in any logical way:
NES side | SNES side
Yellow <-> Black->
Orange <-> Green->
Black <-> Red->
Blue <-> Yellow->
Green <-> White->Ideally, you would figure out which wire carries which signal using a multimeter but mine is on the fritz, so I just trial-and-errored my way through it, which was a pain in the ass, but whatever. It worked in the end. Interestingly and unexpectedly, NES' B-button becomes the SNES' Y-button, and NES A becomes SNES B, which is really fantastic, since the angle of the SNES B and A buttons would make them hard to press at the same time had the naming/mapping been consistent.