Just get a fuse of the same size and ampere rating ... Be carefull that higher ampere rating can damage your amp , and lower rating will blow the fuse more often if not at once (not a big deal of course).

I think your amp's fuse should be a 4A rather than 0.5A , make sure before you put it in your amp ... The rating should be written on both the blown fuse and the fuse cavity.