This place seems as good as any:

http://www.gardenbargains.com/item-300081/10-re-usable-heavy-duty-hessian-sacks.htm?Media=GGOOGLE&gclid=CLryg-OVwKICFRyX2Aod5Cqg5A

These EcoCharlie ones are perfect, and from an ethical company too:

http://www.ecocharlie.co.uk/hessian-sack.html

Well whilst more expensive per sack they at least don't charge 3 times the usual amount to deliver to Moray!