For GBA flash carts, your only real option to find new is the ez-flash IV. All the others have been discontinued a long time ago, the ez-flash IV was discontinued only late last year.
I've got a couple of them and can say they are a great cart.
You can play GBC games, but only via emulation. Some of the early NOR based flash carts could run GB/GBC roms directly when used with an adaptor (the voltage is different and the carts push on a physical switch in the gameboy to switch modes), the vast majority cannot play them directly.
Both the realhotstuff websites list them as out of stock (yet still let you add them to cart). May be worth contacting the store.
http://www.realhotstuff.com/EZ-Flash-iV--OUT-OF-STOCK_p_76.html
http://www.realhotstuff.hk/EZ-Flash-IV-Flashcardout-off-stock_p_76.html
Flashlinker-shop also out of stock atm, but list that they may be back in on the 31st Jan:
http://www.flashlinker-shop.com/ez-flash-iv-minisd-slot-2-gba-gbm-sp-nds-nds-lite-p-443.html
There's links to more sites that sell them if you check back for similar threads in this section of forums (
http://gbatemp.net/forums/gba-hardware-devices-and-utilities.13/), odds are most are now also out of stock.
Good luck in tracking one down