What about the Purificup system. It's less expensive then the Steripen, but I don't know how easy it is to use on a stream.
http://www.amazon.com/dp/B005PULY2I/...ONME4Z1K&psc=1
And there's the Lifestraw that looks really easy to carry.
http://www.amazon.com/dp/B006QF3TW4/...I3VIX4TT7AKKON
Randy