This online store has a fantastic range of pottery supplies. I have spent about 200 on glazes, tools, and mediums, and I will continue to shop here. It would be good if they provided more information (about firing ranges, etc) as not all of the information is on the page for each product, but they do a pretty good job of letting you know what you're in for.
The shipping is expensive, so best to save up and do a big order.
