The best tartan search engine in terms of having the most complete record of tartans, and accessing information about those tartans, is the Tartan Ferret at the Scottish Tartans Authority site. This will allow you to search the complete International Tartan Index.
http://www.tartansauthority.com/Web/...rchintroc2.asp

However, if the purpose of your search is to view the different tartans in their various color schemes, as they are available from different mills, you will want something different. In this case, I reccomend the search on tartanweb.org. This will show you actual photographs of tartan samples from most major Scottish tartan suppliers, and let you know what is available in terms of fabric weight and color scheme.
http://www.tartanweb.org/

Lastly, if you know in advance which merchant you plan on buying from, a lot of individual merchant sites will have tartan searches showing what is available from them, and you should use those, as not every tartan merchant will be able to order cloth from every woolen mill.

(For example, the tartanweb.org search has Batleys, House of Edgar, Lochcarron, Martin Mills, and Strathmore. If you were shopping at the Scottish Tartans Museum gift shop, I'd reccomend you look at our own availability list, http://giftshop.scottishtartans.org/material.html, where you'd notice that we don't order cloth from Batleys or Martin Mills, but we do order from the other three listed, as well as Ingles Buchan; plus we have the option of custom weaving tartans that are not generally available.)

Aye,
Matt