V belts are primarily described by two dimensions: the width of the belt at the top, and the length around its perimeter (which unfortunately can be measured in several different ways: outside, inside, and effective lengths). Common or garden V belts are A, B, C, D or E section. A section is 1/2" wide, B section is 21/32" wide, C section is 7/8" wide, D section is 1 1/4" wide, and E section is 1 1/2" wide. Depth increases approximately in proportion to width.

There are less common V belts of different widths from the above - usually they are narrower, and are made as automotive fan belts. There are also various qualities available. Do not use a washing machine belt as a fan belt (and don't ask how I learned that lesson).

Here is a listing of all the standard off-the-shelf V belts available from industrial suppliers:
http://www.vbeltsupply.com/conventionalvbelts.html

Don't despair if it turns out to be a fan belt type (often 3/8" wide, and thus not a common industrial type). Just about anything is available from somewhere.