Most are 5mm x 20mm. confirm this. Then you can order anywhere. amazon is probably easiest.
PS Not sure what you mean by the cap. I think maybe the cap is the plastic piece that holds it into a panel mount. Unfortunately, that may be more difficult since the fuse holder is mounted in the panel If you can open it up, figure out from part numbers who made the fuse holder, you can probably not buy the cap but can buy and entire fuseholder and just use the cap.