Once you have a set then you can go into any tile shop, buy a few marble tiles and take them home and cut them into tesserae. You'll never run out of material to use as you'll always have the tools to cut it with. The drawback is the marble you end up with can have some sharp edges. Marble that has been cut into rods (typically 10mm – 20mm width) are a lot easier but you need to budget for the additional cost of getting rods. A good tip if you're using a lot of marble is to cut everything you need then hire a cement mixer for a day and chuck it all in with a bit of kiln dried sand for about 4 hours and this will smooth them off nicely.
Sharpening carbide edges preferably needs to be done by a specialist but the steel edges are fine to do yourself. Again there are plenty of resources out there to learn how to do that.
Balance the extra cost of using pre cut tesserae against the reduced cost of buying from a tile merchant but then having to take the time to cut it all. if you can get rods then it will save you a lot of time.
So, if you use stone and you are likely to use it for any length of time then yes, it is worth investing in a set. Just look at it carefully first so you don't end up with an expensive set of ornaments!
If you have any questions then please do get in touch with me, email; [email protected]