Welcome to the board Crex, interesting problem you have there.
The only time I have seen the kind of glass you are describing is on a Cristolian - a picture painted on the inside of a piece of convex glass and then framed (usually antique). Finding an oval replacement may be tricky but luckily you need a 'standard' size, have you tried picture frame suppliers? if you are in Britain you could try phoning this bunch:
http://www.wessexpictures.com/ if they can not help they may well know who can. Otherwise try a framing suppliers in your own country.
You could make one/have on made, you would need to have a sheet of glass slumped in a kiln over a suitable former.
John
It looks like Cristolian may be spelt Crystoleum, but there seem to be several spelling variations in use.