I bought 2 G6 Lite's from RealHotStuff.com and they were a great store.  I liked the service and would buy from them again.
A lot of people have bought M3's from KickTrading as well with good reviews, and I believe they have the Supercard as well.
I don't really like the Supercard since its GBA  compatibility is not as good.  I'd recommend one of the M3s or the G6 personally.  I really think they are worth the extra cost, but that's just my .02.