All,
I blew the fuse in my tail battery a few days ago (shorted while trying to connect the battery to the water pump to add tail ballast with some alligator jumper clips). Couldn't find the fuse in local auto parts stores (the normal sizes were all slightly wider in diameter. I went to an European car specialist - they didn't carry the fuse either, but showed me how to use the code on the fuse to source via Amazon. (I had emailed Vitek at Blanik as well.)
I was able to order a 10 pack of the fuses on Amazon for about $7.00. The fuse "code" is F10AL250V (10 amp, 250 volts) and if you search for this on Amazon you will find many options. The glass fuse dimension is 5mm x 20mm. I received the fuses today and they did fit exactly in the battery pack.
You might want to consider ordering a few spares to have on hand since they don't seem to be readily available in US auto parts stores. Note that the avionics still work with the fin battery inop thanks to the FES batteries. (But the fin battery is needed for ballast/CG purposes even if inop!)
Finally, here is the info Vitek provided from the factory for some other sourcing options: