If someone has gone to the trouble of sawing a piece of stone in the last few years, it may well serve the purpose. If you don't have a piece of stone in mind:
Look in local directories under
Quarries
Stone masons
Memorial masons
Stone merchants
Work your way through in that order. Going direct to the quarry is the best bet, especially if they have a stone saw. Tell them you are looking for a few bits to learn carving on and ask to rummage in their off cuts pile.

Don't expect this stone to be free but do bear in mind that you are not making work.

The 'off cuts pile' is the novice carvers best friend. Only order specific pieces of stone if you don't feel comfortable 'rummaging'.