What size are you looking for? The only basic difference between a sander and a grinder is what you are sanding or grinding. Usually if "sanding" wood it is a sander. If "grinding" metal then it is a grinder.
For starters whatever you want to start off with is directly proportional to the size of you pocketbook.
At the very low end is a 4 X 36 inch model. Look at Harborfreight or Grizzly for one of these. Sears and some hardware stores like Home Depot or Lowe's carry only 36 and 80 or 120 grit.
You can find a 6 X 48 at the same places I mentioned above. The same deal with the belts.
Next is a 2 X 48 inch model. Kalamazoo comes to mind, check K&G. Texas Knifemaker's Supply also has a model or two.
The most common size for knifemaking is the 2 X 72 inch model. Grizzly has an entry level priced model for about $300. Other models for not "too" much more is the Pro Cut, Coote, etc.
Higher end models are are a Wilton, KMG, Bader, Hard Core, etc. These range from about $1000 to $2000.
I started with an upside down portable belt sander clamped in a vice and now have a variable speed Hard Core and seriously looking at a KMG. A fella can't have too many grinders. Be sure to let the missus know that little known "fact".
Craig