Yeah the price has steered me away from an NES RGB mod for sometime as well.
In response to your original question, the Nintendo brand cables are probably your best bet, but there are other options.
Custom hand made cables from this guy out of Poland with high quality shielding and no composite end for low interference. Decent price as well. Ebay link:
https://www.ebay.com/itm/High-Quality-N ... SwBkRaIHkUChinese cables that also don't have a composite end. Basically the same as the cables above without the shielding. I use these for my GC and N64 and they give a better output than the Pelican brand universal cables I used before. Super cheap too. Ebay link:
https://www.ebay.com/itm/S-Video-AV-Cab ... SwFXNZd4oRAlso Castlemania games has HDRetrovision cables in stock right now. If you are looking for SNES cables specifically and have access to component inputs, this is the best option. You need to mod the N64 or use a PAL GameCube for those systems though. I use these on a CRT and they look fantastic. Worth the somewhat expensive price.
https://castlemaniagames.com/shop?olsPa ... etrovision