To be true to the style, I'd have to recommend sticking with the English bitters. My preference would be for the ESB style--even though it's a bit higher ABV--but I think you'll find the selection to be more readily available. Fuller's is probably my favorite, but I'll also pick up Redhook ESB from time to time.
This is probably the easiest and least expensive style for homebrew, as well. If it wasn't so expensive to ship, we could work out a deal....
