Amazon has 'em.
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Most Electrical supply houses, Graingers, some automotive stores, Parts Connexion, as much as I hate to mention.....Radio Shack. Need 3AG, Slo-Blo, 250V, 1/4 or .25A fuses. (ie: http://www.alliedelec.com/littelfuse-0313-250mxp/70184149/) You may want to have your tubes tested, if you haven’t already.
Above are links to both most common sizes 1/4" by 1 1/4" and 5MM x 20MM
Best of luck