Not too sure what you mean... the writing surface is abnormally stiff? Or you must apply a lot of pressure for your input to register?
If the second case, something might be wedged on the side of the screen. Try brushing around it to remove anything.
If nothing else,
http://www.dealextreme.com/details.dx/sku.3245 - replacement lower touch screen, 2.60 USD, free shipping. Having personally ordered one, I have nothing but good words, except that the writing surface is not polished and ultra smooth like the normal touchscreen.
http://www.dealextreme.com/details.dx/sku.21907 might be different, and it is a penny cheaper. However, using a screen protector (which you should be using anyway!) fixes the issue perfectly.
Only other problem is that these dont come with the touch screen border or the sticky tape to hold it to the lcd, so you must reuse them.
Edit - as for replacement, you gotta do it yourself (or find someone that does them).
http://www1.dealextreme.com/productimages/...905_1_small.jpg are the screwdrivers you will need to disassemble the DS, and we at gbatemp have a disassembly guide stickied in one of the DS forums.
