There's two good choices for you.
First is the Supercard DStwo, it's a bit pricy but it's game compatibiliity is pretty much perfect thanks to it's anti-piracy circumvention. It has a lot of other features too, but it's still the best at just playing DS games.
Second is the Acekard 2i, it's unofficial AKAIO firmware is brilliant, it has it's own anti-anti-piracy which improves compatibility, and regular updates.
They're both SDHC, and both have great compatibility.
If you're in america, I suggest www.realhotstuff.com, if not, try gamekool.com