I have found that Farnell sell the 27800 EPROM (8Mb 512Kx16bit) for just £7.84 +VAT. I think they have a US site too. The site says it's 40 pin, but the PDF that is supplied with it says it's 42 pin? (perhaps one is OTP? or it's a typo) If they are the 42 pin variety they should just drop onto the 2 chip cart blanks that are availible from Best Electronics.
I am pretty chuffed that I have found a potential supplier of cheap eproms, hope it's of use to someone else. If they turn out to be no good I guess it's back to the design your own cart's again













