As njsurf said, you can get 4A slo-blo fuses at Radio Shack, your local hardware store, wherever. It's a common part.
Fuses blow when the current passing through them exceeds their rating. Usually, this is because something shorted out (Ohm's law -- current increases as resistance decreases). With tube amps, the first component I would suspect is a tube, and probably a power tube at that.
Get a few new fuses -- you're probably going to blow at least one more figuring out the problem. Replace your fuse, take all of the tubes out of the amp, and turn it on (don't worry, you won't hurt it running with the tubes out -- it's just a box of open circuits that way). It probably won't, but if the fuse blows again, take it to a tech, as there's something internally wrong with the amp which needs to be fixed. If the fuse doesn't blow, put the tubes back in, one at a time, starting with the rectifier tubes (put these back all at once), then the preamp tube nearest the input jack, then the power tubes (replace these all at once, too). Turn the amp off while plugging tubes in.
At some point in this process, you'll blow another fuse. Replace the last tube you plugged in (replace the set if it's a power tube). A TR has three rectifier tubes (hence the name), so if replacing those blows the fuse, you'll have to get another one and swap them out one at a time until you find the culprit. Unless it's safe to power on the amp with only one rectifier tube -- I don't know for sure, perhaps someone else here does.
