Pretty much all flashcarts all do the same damn thing nowadays.
Except for the REALLY low-end shit ones.
Acekard 2 can be had for just under $12 Here
And that's with free shipping too.
(But keep in mind, these guys may have free shipping, but it's slow as fuck. Expect between 10 and 20 days)
As for a MicroSD card, it all depends on what you're doing.
For just games, a 1GB will hold a good deal of 'em, 10 at the very least I'd say. (DS games can vary in size between tiny little 8MB games all the way up to 128MB)
2GB will hold at least 20. (More realistically, because MOST games don't hit the 128MB mark, it'll hold more like 40+)
4GB will hold PLENTY of games, and more importantly leaves you with room for music, movies, etc, if you decide you'd like that sort of thing on the go.
As for that, you can grab a 4GB Here
for about the price of the Acekard itself.
These guys are sister site to the other place I linked to.
So, again, free shipping, but slow.
And if you need a MicroSD card Reader to go with it: I love these
THEY'RE LESS THAN A DOLLAR, they're fast, and indestructible, I keep one on my keyring.