You should also investigate if the cards do work with the nds lite. Rumor tells that some cards work on the normal ds but not on the ds lite. Dont know if this is true. Currently i use a r4 with my ds, before that i used a m3 with that m3 it was almost impossible to hold the ds with things sticking out in the back and front of the ds.
In the netherlands you can find the r4 being sold on the weekmarket next to the vegetable stands :-) Dont remember what i paid for it but was is under the 10 euros.