Ames sells the inner fenders for the 66-67 and says they will fit a 64-65. The difference is that the 66-67 has an indent to accommodate an 11" brake booster. PY sells the same inner fender but does not say that it fits a 64-65.
Have you tried Frank's Pontiac parts?
http://www.frankspontiacparts.com/usedparts/index.htm They normally have a good selection of used parts.
Other places to check are Billions and Trillions.
http://www.billionsandtrillionsinc.com/ , Potomac Classics
http://www.potomacclassic.com/
I have some other places too but they don't have web sites and their phone numbers are at home. If you would like their numbers let me know and I will post them tonight when I get home. I found Potomac Classics and the ones that I have the phone numbers for from dealing with places like Ames and PY. When they don't have the new parts you need they have a list of used places to call that they recommend and have great reputations. I have used a couple of them and they are very reliable.
Dale