.303 surplus is out there, but it is really difficult to find. The same goes for 8mm and .30-06 and as far as I know the Japanese stuff is uncommon surplus wise aswell. Your best bet for surplus would be the 7.62x54r round for the Soviet SVT and Mosin series of rifles. At the moment the surplus ammo isnt exactly common but it trickles in every so often.
That being said .303 and .30-06 ammo can be bought almost anywhere ammo is sold, including Canadian Tire. This is however factory ammo and will usually run around 1$ a shot or more. Sucks but it beats having no surplus and no factory for your Mosin/SVT.
Of course the best option if you plan to consistantly shoot any milsurp rifle aside from the SKS is to reload. In the long run you will always have "match grade" ammo at your disposal that will be non corrosive and inexpensive - its win win
A standard WW2 dated No4 Mk1 Lee Enfield rifle will run anywhere from 325-500$ depending on year, condition, country (Cdn ones seem to be more expensive here) etc... They can be found everywhere right now through our site sponsors.
A run of the Mill Mosin 91/30 or M44/M38 carbine will run you anywhere from 150$ - 225$. An inexpensive rifle however they are highly underappreciated and are some of the most accurate, robust rifles out there. Especially for that price. Again they can be found eveywhere through our site sponsors.
An M1 Garand is where things get pricey. Typically from what Ive seen prices start at around 800 and go up. I have seen some deals for cheaper ones, but typically there not a starter rifle as they are expensive. To find those your best bets are gunshows and online sites. A very fun rifle to shoot however!
The K98s are another option. Right now Russian captured pieces range from around 400-500$ depending on condition and markings (post war a lot of nazi stamps were ground off). These are excellent, extreamly accurate rifles with lots of history. The only downside is the ammo which where I am at times is very expensive. 35$ a box for factory wasnt uncommon.
Japenese stuff I have no idea, someone here will chime in
Overall it depends on what you want. What country? which years? what theatre of operations? etc...