Dawn - try Forbidden Planet as they usually stock them, ours in Derby had them a couple of weeks ago and I need to grab a handful to put my sis-in-law's COS base set into her binder for her Chrissy pressie so I can check if they still have them.
Give me a shout if you can't find them where you are and I can see if I can pick some up for you here!

Oh ... and they normally sell them individually rather than in packs of 10, so it can cost quite a bit if you want loads of them.

Jules, what COS set? WOTC?
Sorry John ... my bad ... I meant to write 'Sorcerer's Stone' set!

I have the binder (courtesy of Tom - thanks Tom!) and I have the base set from Roz (thanks Roz), so all I need to do is get the Ultra Pro's and that's sis-in-law's pressie done and dusted!

You're right on the postage being high if you buy them by the 100, but they
are heavy and that in turn pushes up the price.
The first of the two links posted - chasecards.com - are really good and the seller packages very well.
The second seller is someone across town from me and I keep meaning to give them a ring to see if they'll sell over the counter to the public or if they're strictly mail order/ebay sellers. I don't know much about them but they might be the cheapest ever for me if I can just hop over in the car and fetch them!
