Have a 20 plus year old microwave convection oven that took a 125v 15 amp fuse which is hard to find so can I safely use a 250v 15 amp fuse as a replacement?

Yes you can. The critical thing is that amps match. All 250 volts says is that it will work up to that value. You wouldn't, for example, use a 20 amp fuse since that could allow higher currents than the microwave could handle. The fuse blows because of excessive current.