with how thin the phones are now, I really haven't had much problems just buying a phone case with a keyboard on it
these are for iphone, but you can find one for other sizes with enough looking
Amazon.com: Buletooth Wireless Rubberized Hard Shell Slider QWERTY Keyboard Case for iPhone 4: Cell Phones & Accessories
Amazon.com: Dobi Design Apple iPhone 4 Bluetooth Keyboard Case with Stand: Computers & Accessories
edit: forgot to mention but should be obvious... phones that are curved on the back have a hard time with these cases...