We've bought most of our coins from Oakcoins (www.oakcoins.com). Join up to their mailing list and you'll get sent free postage from time to time which makes it ace.
Clearly, make sure you keep it under £18 to avoid customs charges.
A couple of friends of ours run Geotees so we use them quite a lot. I've often found it's just as cheap to get official geocaching.com merchandise from the US too. Hobbycache used to be great for this, but they've let their site go



