Where in Liverpool can you buy an r4i card for the 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.