You're looking for a 27C301, any numbers before or after that aren't relevant. They are obsolete but you can still buy them from ebay secondhand. The 27C1000 seems to be identical.
http://www.ebay.co.uk/itm/160618159397 for 10, which is mysteriously roughly the same price as getting one or two.
Annoyingly this EPROM doesn't seem to be supported by the standard Willem Programmer, which is what i'd use for burning just about everything similar. I might be wrong about this so it'd probably be a good idea to check it out, but the 27C1000 or 27C301 don't seem to appear on any supported device list for the Willem that i can see.
This seems to support the 27C301 but its getting a bit pricey
http://www.ebay.co.uk/itm/160554620185I guess what you'd have to do is read one of the EPROM's you have and work out exactly what format it is. Then load the file into an audio editing program and create a new file with exactly the same layout and file characteristics, but with different sounds. Then try to burn a new EPROM with that file.
Alternatively you could get an Akai S20 for about £60 which you could load any sound into easily and which produces better sounds than the TR505 when bent anyway!