You can get the Moebius kit in this country from a few model stores.
Modelzone is one, Comet miniatures another.
Also ebay is a good source for these models.
AFAIK the Caswell kit is the only conversion kit on the market for this boat, it's that or it's DIY I'm afraid. If you know someone in the States who can act as a go-between, then that might help lower costs at your end, if you get my drift.
Large GRP Seaview hulls are made by Deboer. Lovely hulls from what I've seen, but the price will make you cough and splutter. Rick Teskey in Canada does larger Seaview hulls in various sizes, and supplies both four and eight window versions. The prices are lower than Deboer, but will still be considerably more than a Moebius conversion. No website for Rick, but he does check in over at the Sub Committee and Sub Driver forums from time to time.
http://www.deboerhulls.com/Seaview.htmhttp://s181686668.onlinehome.us/phpBB3/index.phphttp://forum.sub-driver.comAndy