Correct me if I'm wrong, but it doesn't appear that there are any websites producing/manufacturing NES carts of homebrews. Nor does there seem to be a large number of original homebrew games (versus hacks and demos).

And that is very interesting and disappointing. There are numerous Atari 2600 homebrews (including Skeleton+, which I wrote) along with several websites producing them on demand (AtariAge and PackRat VideoGames to name two). Naturally an Atari 2600 cart is simpler than a NES cart (one EPROM and a PAL for bankswitching logic), but the homebrew 2600 market is now large enough that places have gone to using new PCBs and cartridge shells rather than salvaging.

So what's holding everyone back? Mappers? Come up with a simple but effective mapper (e.g. just bankswitching) that can be done using a PAL. Lockout chips? Slap a 'scope on a cart and reverse engineer it, or copy one of the workarounds that has been done before.