There's a guy here on Ebay selling sugar beet seed from Turkey - 100 seeds for £1.55 plus £1.99 shipping. He seems to have a good reputation, and the seeds are supposedly heirloom so we could save our own for another year.
I've bought some, but it's too late to sow them now so I can't vouch for their quality. Still, at that price it's maybe worth the risk...?