HP has a wonderful online parts catalogue; search for your model number and it should give you options for all the processors HP offers for the model number, then do a separate search for that HP part number (in a search engine) to find a decent price for one.
HP Part Surfer
Also, find your mainboard part number and see what other models use this mainboard. You can see what other goodies you might be able to plug in. I found a bluetooth module that would plug into my G7, even though G7s don't have a bluetooth option.