Ammo is available. Just a little expensive. Plus you have to pay the hazmat fee for shipping.
Your best bet is to reload because it will really cut your costs.
Buffalo Arms has everything you will need to roll your own, or if you are not interested in reloading you can buy loaded 310's. If you speak with Dave Gullo you will be in very good hands. Caution: The bullet is a 'healed bullet' design-learn what that means. A hint is the typical 22 rimfire long rifle- it's a healed bullet design. Parts are not commonly available, but they are out there.
They are at http://www.buffaloarms.com/about.htm
If you have an unaltered Cadet, you are lucky that was issued to New South Wales. You probably have some marks on the buttstock that will date the rifle, example: 9-11 means Sept, 1911. Don't mess it up. They are worth better than $500 in original condition.
Want to learn more?
Go here http://www.martinihenry.com/links.htm