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

Amazon has 'em.

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.

Home depot has plenty of them.
Radioshack seems like history already with most of the stores closed and it's hard to find one within 200 miles around.

http://www.mouser.com/ProductDetail/Littelfuse/0313250HXP/?qs=sGAEpiMZZMtxU2g%2f1juGqbdCHRI5TALuijmW...

http://www.mouser.com/ProductDetail/Littelfuse/0229250MXSP/?qs=sGAEpiMZZMtxU2g%2f1juGqbdCHRI5TALukyw...

Above are links to both most common sizes 1/4" by 1 1/4" and 5MM x 20MM

Best of luck

Peter

I pm you.