There are lots of sources for glass and acrylic domes to fit anniversary clocks and dolls, but one to fit the needed dimensions might be difficult to find. If you don't have any luck Googling 'glass dome' and 'acrylic dome', you could have an acrylic fabricator make a rectangular box slightly larger, and the put the whole thing - oval base and all - on a rectangular base matching the new acrylic box. You also could build a rectangular enclosure out of Museum Glass, which would provide 99% UV filtering and anti-reflection, and would probably cost you less than paying an acrylic fabricator.