I would look at the Coys web page. This company have a number of different hypoxic chambers available in the shelf or they will make a bespoke chamber for your needs. These chambers also come with environmental scrubbers to remove carbon dioxide and moisture from the air. These are closed chambers so will use little nitrogen when running. Just need to keep an eye on the carbon dioxide concentration. Biospheric's also produce a chamber, this is an open-atmosphere chamber, essentially a leaky perplex chamber with an o2 and CO2 sensor. This chamber needs to be plumbed into a nitrogen generator as it uses a lot of nitrogen. The more mice you place in the chamber the higher the CO2 and humidity, this chamber uses the passage of nitrogen to control these parameters. Removing bungs in the chamber increases the gas flow. I have used both with equal success.