N.H.
M.,
You can rent a helium tank at Arnie's off I-10 and Studemont. You can buy a small one at Party City for around $20. Arnie's has all kinds of party stuff and good prices.
I made a carousel cake for my son's 2nd birthday and it was cute. It's basically a two layer cake with peppermint sticks for the poles and decorated animal crackers for the animals. It just called for a paper roof.
Anyway, hope this helps.
N.