I used to buy IRC's in the US for requests to Canada or the UK. But, they were more expensive and I was always afraid of that one celeb who didn't want to be bothered exchanging it for postage. Also, they are hard to find at my local post office. So I contacted the Canadian and British post offices and ordered postage over the phone and had them mailed to me. I only send using standard size letter envelopes. The postage you see below is for letter size. So if you plan on always using larger envelopes. Just ask them what postage would be when speaking to them. When I send the my request. I just put the stamp on the return envelope and your good to go. And I think I have had better success doing this.
Canada: I order 98 cent (Canadian) stamps in books of 6 stamps. It costs me $5.98 (Canadian) per book and they DO NOT charge shipping.
UK: I order 90 pence stamps in whatever quantity I need. I usually order between 10 and 20. Since they DO charge shipping and it doesn't change much whether you get 1 or 20 stamps.
You also are going to need a credit card to order. Here are the phone #'s:
Canada Post # 1-800-565-4362 (A toll free call)
UK Royal Post # 011 44 131 3167483 (a toll call to the UK)
Hope this helps. If anyone has any questions. Just ask!