they are on eBat for about $5.00 each. Limitron are super fast acting fuses. No doubt they are in the VTL to protect the B+ transformer windings. Regular fuses have to melt to break, Limitron blow on over amperage instantly. Limitron is the brand the Fluke uses, so there must be a reason.
My VTL MB-450 have KTK-2 two amp rated, KTK2-2 is 2.5 amp rated and that is a 25% amp increase....not a good idea since the amp is rated at 2 amps.