Anyone know a vendor that might sell SummerCart in amateur quantities? Doesn't need to do that specifically, but maybe there's custom low volume board makers, like with 3D printer economics.

Are you looking to make homemade homebrew carts? If the M64 carts are anything like ModRetro's chromatic carts, the boards will be flashable. ModRetro has also stated they're planning on open-sourcing the design, so if we're lucky there could be a large variety of people making flashable blank carts in the near future.

I'm really looking forward to the Epilogue 64 Operator. I have the GB Operator, and it's been amazing to be able to flash romhacks onto carts and play on bona-fide hardware.

As open-source hardware they are on AliExpress no problem