Not sure what fits a dell and what won't, but if you haven't looked already
http://www.newegg.com/ProductSort/Categ ... ategory=11
Another good site is zipzoomfly. I ordered almost all my PC components from there.
edit: not that familiar with dells, but do they use their own proprietary made fans, or do they use some other fan? Last time I opened a dell, it was rediclous lots of custom parts to prevent you from upgrading yourself, lots of cheap plastic tabs that if attempted to open, would just break.
I'd take a look at your heatsink(cpu fan) and see if theres any distinguishing marks on it, manufactuerer. If it says dell on it, chances are you'll either have to send it out to them or order the part from them.
edit: Found dell's "community forums"
http://forums.us.dell.com/supportforums ... =dim_other
maybe you can find some info there