A lot of people seem to be recommended the Acekard DSi card at the moment. I just had a look and those cards are compatible with the DS, DS Lite and DSi, so the one card works for all which is good.
And the prices seem about the same as the R4 and R4i cards.
I've also discovered a great Hong Kong site http://www.dealextreme.com that reminds me so much of Lik-Sang. They are selling DS flash carts, plus lots of other stuff for many different systems. They also have some great looking handheld emulation systems worth looking at, and all at great prices with free shipping.