Afraid I have not kept up with flash cart sellers. The EZFlash people have a dealer page
http://www.ezflash.cn/dealers/
Forgot about the GB/GBC stuff last post.
GBA flash cart don't natively play GB/GBC games save for one very old one with a special bridge device. It is effectively a different piece of hardware (the small black lever/switch in the slot -- that activates it, indeed if you ever put something GBA in a GBA/GBA SP and it looks squashed like GB/GBC mode check said switch).
There are however emulators. Originally only the gameboy was emulated, typically via an emulator called goomba, and later the GBC (and GBC mode for those games which had a backwards compatible mode*) via an emulator called goomba color. Goomba color was quite stripped down compared to plain old goomba (or some of the forks it had) so some opt for said same for those things which can work here. Quality wise there are better emulators out there but most games will play just fine and if you get good on such a thing you will be able to go right onto original hardware and will be just fine.
The flash carts which offer any kind of support for this will have a version of these somewhere in the mix.
You will not be doing link cable with original GB/GBC games using said emulators.
*5 types of GB/GBC ROM.
1) Plain old GB and only GB. Usually made long before the GBC as a twinkle in some designer's eye.
2) Super gameboy.
http://loveconquersallgam.es/post/2350461718/fuck-the-super-game-boy-introduction for that one. There was a device series which attached to the SNES and allowed a few processing extras, few games did much in the end but it is a thing. For me the multiplayer bomberman version made for it is one of my favourite examples of that game. No GBA emulator emulates any functionality here as it more or less means emulating a SNES on top of things and the GBA barely managed more than proof of concept SNES emulation.
3) GBC with backwards compatible mode. Had a GBC mode with some colours, possibly some less slowdown and a few other things but mostly just the colours for those things which supported it.
4) GBC and only GBC. Used the extra hardware of the GBC to do games. A few people once hacked games to bypass the checks -- they glitch massively and crash in the best cases.
5) GBA perks. Normally I don't count these as it is a simple check and no processing extras but owing to something I cover shortly there is a bit of nuance. The Zelda Oracles games are the main things people look at here, though Shantae had some stuff here. Goomba color will emulate this detection method if you wanted it, though you can also patch it in. Speaking of Shantae it also upped the brightness in GBA mode (the original GBA screen being what it is) so there is a hack which will give you the bonus content while keeping the brightness normal.
https://www.romhacking.net/games/2769/ for more on those.
Many GB only era games got enhanced ports to the GBC, typically with the name DX added onto the end of it but not all of them. There were also a few things that were GB and GBC in Japan but turned into GBC only when it made it out of Japan.
