The Private Reserve or Schmidt Brand make excellent replacements for both the Cross and the Parker type refills. Several vendors here sell them.
One tip, be sure that the name, and point are printed on the barrel of the Cross type refills. At one point, Private Reserved didn't imprint this information onto the refill. Keeping up with the unmarked refills was a major hassel. I THINK all of the IAP vendors now have the imprinted PR Cross types. This was not always the case, so I'd ask before ordering.