you'll want to stay away from supercard slot-2 products. you will spend more time debugging, hacking, and researching than enjoying homebrew and roms. the m3 lite perfect (not the pro) was by far the best slot-2 i have ever owned. there was a gba sized shell for it that shipped with the pack for a while. its harder to find it now as m3 no longer makes it. i've seen a few shops that still stock it with the lite size shells for around $90. you could easily mod a real gba case to hold it, if you wanted to.
the ez iv is probably the next best thing but lacks the bells and whistles of the m3 lite (real time save, speed hacks, emu support, movie support, audio playback support, password protection, cheats, and more) ez iv is what the 3in1 was built from and is a dedicated slot-2 solution that works off mini-sd cards. it also has psram and nor so you can flash and play as a normal gba cart.
if you can find one the flash2advance and flashadvance kits are really nice. they will only hold a few files at a time but they last for years. my f2a is going on 9 (?) years old and still works like a charm. the fa require a printer port for flashing and the f2a (usb linkers) require a usb port and an original gba.
when you have 100 posts you can gain access to the trading forum here at gbatemp. you might be able to pick up an older gba kit from there, but expect to pay for it because most people know what they have. try reading the gbatemp reviews and also check gameboy-advance.net for more information.
no one stocks the old gba kits any longer because they are not made. the only one that is still in production would be the ez iv. last time i talked to m3 they said they aren't making anything or writing software updates. i'm not sure about supercard, i think they are still supporting their old kits but they no longer make them (might want to check their forum for more information).
-another world
