Get a Suuntu altimeter wrist watch (models range in price from $200-500). Most have similar functions altimeter wise.

I've read pretty good things about them (just bought one), but have not read much as far as continuous reliabilty as far as cycle computers with altimeters go.

If you don't like to wear a watch. or not when you ride anyways, you can strap it onto your bar if you have space (they sell an accessory just for this).