In the UK, small disposable cylinders (too small for a normal latex balloon) are sold, but anything larger is rented out. Typically you pay for 1 month's rental but can extend that at cost.
The best prices here for Helium are online - I use www.balloonhelium.co.uk when I need Helium.
Hydrogen though is much, much cheaper. In the UK to get hydrogen you need an account with BOC or Air Products, which you can get by filling in a form at the local BOC/AP depot or agent. I have a BOC account and the total cost is made up of a cylinder rental, a delivery or collection fee, and finally the gas itself.