where can I buy a 1/4 amp slow blow fuse?

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.

BTW:  Some companies list their 1/4A fuses as 250mA, so- don't be thrown by the terminology.

Amazon.com