This is the cheapest source of "B" cells I have seen if you are making authentic battery packs:
http://cpc.farnell.com/gp-batteries/gp3 ... dp/BT00017
About 35p per cell. Maybe, depends on postage.
5 used in a 7.5V LT pack http://www.radiomuseum.org/r/ever_all_d ... _ad38.html
6 used in both types of Grid Bias battery (screw thread or wander plug) http://www.radiomuseum.org/r/ever_winner_9.html
Used in many other LT packs and 60 in a AD3 combo pack.
60 to 98 per "Portable" or "Winner" HT pack! Ouch. I think I will stick to 8 x Alkaline AA for €2, cheaper, slightly more capacity, easier to refill, less prone to leak. Still you might want very authentic packs for a 1920s radio. About £18 per pack though isn't impossible if postage is free on large orders (200 hours to 500 hours operation).