Hamsters don't need to climb bars, in fact they aren't good at height perception so often fall and hurt themselves.. I think running space is more important, and would make him a happy pumba
I'd personally attach a bin cage to your current set up, and have that for running around, it stops you having to mess around with cutting wire bars, as I don't think there are wire cage adapters for rotostak..
Lots of people add a bin cage onto a rotostak so their hamster has room to run, and tunnels.
(btw a bin is just a big plastic storage box, you can get them from most hardware stores)
If you're really wanting a wire barred cage, there are loads of unbranded and branded ones, I'd look on ebay as I'm always seeing them come up on there.. you can usually get them very cheap (usually pick up only)