After some searching, I've finally found a decent UK supplier of 3g jars (I prefer 3g to 5g for some reason). I had been ordering from the US which cost a fortune in postage and usually meant a customs charge. Previously, I hadn't had much luck in finding someone selling less than 1000 in the UK and some 5g jars I had bought previously on eBay had been of low quality.
JT Shop sell 3g, 5g, 10g, 20g, and 30g jars in quantities from 2 jars up to lots of 500, with or without sifters. I ordered 50 3g jars without a sifter for £11.95 and £2.99 p&p. I ordered on a Friday night and they were here on the Monday.
They may not be as cheap as when they come from the US, but I think I'll be getting them from here in future.