Duh, I should ask people who know all the good places to get components.
My old English folder needs 20 x 451 rims wih 28 holes to replace the stock steel ones that won't hold a high tire pressure. The old ones are 20 x 1 3/8" but I could go with 20 x 1 1/8" as long as it's 451. I know that Sun, Alex, and Alienation all make BMX rims that will fit the bill.
I've seen some in American web stores but I'm leery of shipping cross-border, as the brokerage fees will probably cost as much as the rims. Local stores don't seem to be able to order the right size (or they can only quote sizes in other standards than ISO, and can't explain to me how to convert).
Does anyone know of a shop or web store that stocks these and that ships from Canada? Alternately, are there tricks to avoiding the brokerage fees if I order from the US?