Many will say the DSTwo does have a GBA emulator but it is not perfect so a proper GBA flash cart might well do.
About the only proper* GBA flash cart still produced is the EZ4. Not sure where people tend to buy them these days and you will want a miniSD to go with it.
*the EZ 3 in 1 is still readily available but requires and works best if you couple it with a DS slot device.
GB/GBC flash carts... gaming grade ones arguably do not exist (what is available is usually some old stock of an ancient legacy device like the old bung carts that do the job but are expensive and awkward* or devices intended to play little sound DJ that lack proper save and multirom support) but the GB/GBC is emulated very well on everything (goomba color on the GBA and lameboy on the DS).
Saves on the other hand.... I would go custom http://www.ziegler.d...mming%20systems
(link also has a nice listing of some of the GBC stuff) / http://hackaday.com/...gameboy-dumper/
myself but I have seen a handful of devices to try and grab saves. There have been a few discussions lately and some sites to grab some of the old legacy stuff from but I lack the time to go searching right now.
*parallel ports, probably a version of 2000 or XP, custom unstable drivers and more.
... emulators might have something although I have not really gone down that path (PC emulators do have proper cheats
and saves are reasonably portable) http://www.pockethea...c§ion=patch
has some trainers. I am not sure what goes with regards to gamesharks for GBC games run from GBC flash carts.