cheap source for weather balloons?

Hi,
does anybody have a good (and cheap) source for weather balloons that ships
to Germany?
I need quite a lift of several kilos, so I'd need a few more of them.
BTW, since I'm new to this subject, can anybody recommend a good website on
this matter? I would be interested in calculating the ascent rate in
relation to the increasing height. Will an average balloon level of or will
it inevitably climb forever until it bursts?
Yours,
Harald