Are you going to use it for a ONLY DS Lite (Lite sized), or is a normal sized GBA cart that compatible with GBA/SP/Micro/DS/Lite fine? Meaning, any specific sizes?
Other than that, a pretty good one that uses SD cards that is decently cheap is the EZ Flash IV(USES MINI SD). Some people may recommend the M3 or SuperCard, but the Supercard has occasional slow downs and both the M3 and SuperCard are hard to find. The M3 line of GBA carts are hard to find now, and if you do find them, they're the Pro version. It's the same with the SuperCard, they're mostly the Rumble version. Both Rumble and Pro do not support GBA games.
You could probably get the IV from dealextreme (Linky1/2) if you're fine with ordering from China and the shipping takes quite some time. If not, you can get it from RealHotStuff (Linky) which is a great store based in US. I don't really know any other stores in the US that still sell the EZ IV.
Can't be bothered to look for any more M3/SuperCard flashcarts since I didn't find any on RealHotStuff and DealExtreme. It's rather difficult to find them now. The easiest to find is the EZ IV which has pretty good compatibility from what I hear.