You look at a place like wineenthusiast.com and see what they sell wine racks for. Custom-made is a nice feature, as I had to make shelves for my wine cellar because the ceiling was too low in one place for regular racks, and the cellar's too narrow for two racks side-by-side. But most people can make do with off-the-shelf stuff.

Keep in mind, also, that different types of wine bottles have different diameters- I forgot which is which but Burgundy and Bordeaux, for example, one is 3 and the other is 3.5, or something like that. German whites are longer and thinner, and Champagne is wider. Other random wine bottles are also wider.