There are many places where you can buy Fruits Basket from. I recommend buying manga online, while comparing prices, as that’s normally cheapest.

Advice
If you decide to buy online, be careful. When using sites like Amazon and Ebay (especially Ebay) always check the seller’s feedback, and, with Ebay, their description of the item(s). One of my favourites is The Book Depository, as they offer free p&p, and books from them arrive in very good condition, from my experience.

Bookstores
Borders, Waterstones.

Online
Amazon
Amazon (UK)
Ebay
Ebay (UK)
The Book Depository (FREE p&p!)