Eric, you can go to good old Jeff Stubbs in Oundle who is mainly aeroplanes, but will have balsa, I'm sure. Failing that there are model shops in P'borough (or were). There's one in an industrial unit on the outskirts. My son took me there once. if you fancy a nice ride out, go to SLEC at Watton They have absolutely everything including hard woods and exotics. They are basically a huge modelshop who import their own balsa and other woods which you can kill an hour just looking round, easy.

Cheers,
Martin