Search ebay for 3AG fuses.Too much chance of a substandard device.
The fuse is the first line of defense for the amp and speakers
Digikey, Grainger, Mouser, McMaster-Carr
Fuse Manufacturers | Littlefuse, Bussmann, Mersen (fusesunlimited.com)
The extra couple of bux for shipping if you can't buy local is cheap insurance. Buy the exact required fuse. If it goes, there is a problem with the amp. Get it fixed.
In more than ½ century in HiFi, I've never replaced a fuse for failure.
Several because a manufacturer scrimped and installed no name crap.
As normb said, if you have a spare, you'll likely never need it.