In the local Mainhead Aquatics nr me (St Albans), they have a few arowana, but they are pretty expensive. One in there was £2500, and the other was £1200. They did have others but can't remember the price, they were cheaper, i know that much. If you go to www.fishkeeper.co.uk (Maidenhead Aquatics website), it lists all of their shops, there may be one near you. Might be worth giving them a call. Other than that wouldn't really know.
Might be worth posting in the oddballs section of the forum, some people who keep them may be able to tell you a good place to get them from.