It might be worth calling digikey to see if they can get it.. i see 6A of that type of fuse in their catalog, so i bet they could get 12A if they don't have it.
Well, the biggest GDA (time delay) fuse I have been able to find is 10 amp, 250 volt. This is for an amplifier that makes 1,000 watts at 4 ohms. Would it really hurt anything other than maybe blowing a fuse prematurely every now and then to use a 10 amp fuse instead of a 12 amp fuse?
Also, I was able to find a GDB (quick blow) fuse that is 12 amps. Would this be a suitable replacement?
I know it's not directly applicable, but I was having a similar problem finding a slo-blo for my preamp. So I replaced it with a fast-blow of the same rating and it has been going strong for several months now.
Go for the fast blow, the worst that could happen is you blow the fuse. Maybe in the meantime be on the lookout for another source for the slo-blo types...