Itunes usable in one country to another?

I am wanting to give away two itunes gift vouchers to some of my customers, but some are located in different countries. If i purchase them here in England and ship them over to the U.S and Canada, will they still work? or do they need to be purchased in the countries the residents live in?

So if I was going to purchase one for another country, how would I do that? is there any third party sites based in different countries I could use for that?

Okay, perfect. Thanks!