Amazon or e-bay
There are a couple of places you can buy R4 for the Nintendo DS system. You can buy this at Best Buy or GameStop.
IF you mean R4 DS, then you have to buy it.. you cant just download it.
You will have to buy R4i, because R4 doesn't work on DSi.
there are a lot of online sites to sell those r4 cards, just be careful of the R4 clone cards, there are many cards called R4 in the market, if you plan to buy a r4 card for your 3DS, i would recommend you buy r4i gold 3ds or supercard dstwo, both of them are working great on your 3ds.
You can no longer get them on the larger websites like Amazon and eBay.
Someone can buy a R4 DS from the Deal Extreme website. The Deal Extreme website offers a large variety of R4 DS's that can be purchased for any DS system.
One can buy a Patagonia R4 jacket in a wide variety of places. A Patagonia R4 jacket can be bought at a local Patagonia store, or it can be bought at the official website.
What!? A DS R4??? Well i presume you meen 'how do you make the DS have an auto boot R4?' (not possible!) Just buy an R4, theres no way you can have a DS what runs roms without an R4 (or any other flash cards) You can buy them here: http://www.r4i.co.uk/ (for both DS and DSi)
go to dealextreme.com
If you are in Hong Kong, you can buy it in Mong Kok at some electronic store. EDIT: If you live in Europe, google R4 Mania
Nintendo ds and an r4
Pacific mall