One of my credit card issuers has an option on their website to "request sales slip copy." I've never had do to it, but I don't think they charge for this service.

(Taking it a step further and disputing the charge just to get the receipt would be a tad dishonest, though, IMO.)

Good luck.

p.s. would your a/p department not accept the credit card line item along with ticket number and fare paid as proof?