If you are just learning the shaping and finishing wood knife handles, I would suggest not buying wood.
Just about every town has cabinet shops. If you ask politely for some cut offs for practice pieces they will usually give you several chunks for free.
This way you don't have to worry about cost while learning to shape, sand and finish handle material.
If you can't find good dry wood locally, let me know and I can put together a small package of practice pieces for you. You just pay the postage.
Using natural wood it is very important that it be dry. For hardness see if you can dent it with your thumbnail.
If you can't, it should be hard enough.
As long as you use good dry wood and finish it properly it should hold up good.
When you get to the point where you can make a decent handle, then think about buying handle material.
Professionally stabilized wood will work a lot like normal wood as far as shaping and sanding. Will be easier to finish.
So....if you learn the basics through trial and error first, then you won't have to worry as much about ruining expensive pieces later.