Ebay has PCBs, Sean Kelly has reproduction cart shells. If you're making your own dedicated carts, you might be able to get both from someplace like Packrat Video Games (who get shells from Sean and PCBs from Harvey DeKlein).
I don't know why anyone one bothers 3D printing shells when Sean's reproductions are perfect in every way. As long as you're not making a competing multicart, he's fine selling them to you. He only requires that there is a minimum order of 4 shells.
Harvey DeKlein's boards are really the way to go. No soldering, they can be programmed at anytime with the edge connector on the PCB with a standard programmer and a special connector.
As for legality, its fine to make personal carts of any game. Any original GCE game can only be sold at cost (per Jay Smith's request, the owner of the rights to all original era Vectrex properties). Any homebrew game must have the permission of the developer. In most cases, they are selling the game themselves, having someone else sell it for them, or wish that their games not be sold. Please respect their wishes.