I have a CycloDS and recommend it if you do not mind the price ($45-$50).
My girlfriend has an iTouch DS, a variant of the R4, I believe. The only good feature on it is that the Mini SD card is spring loaded. The card offers no features to speak of other than play games. After purchasing an EZ Flash 3 in 1, I found that the iTouch does not support most accessories. In short, avoid it if you want to do anything other than play games that come in pre-patched and trimmed ROMs.
As for where to buy, I have never had issues with Modchip Central or Real Hot Stuff.