Where in stores can you buy the R4i card for the Nintendo DSi?

R4 and similar Flashcards are banned in the UK. you can they online. If you can wait some times, you can buy online from China, it will arrive you from China in 10 business days by normal shipping.

Though it will take a longer time, by much cheaper than local shops.

I recommend gamehouse88.com, it is a professional r4i and other flash cards reseller.