I like Roadrunnersports.com because you can search by size. A quick search showed a few styles of pants that come in petite sizes, including a pair of tights in their own brand. http://www.roadrunnersports.com/rrs/products/RRL895/

Personally I like all styles -- tights, looser warm-up pants, and in-between styles. This is my current favorite pair from another online store: http://www.runningwarehouse.com/descpageWRA-SWBSTNP.html. It's sort of an in-between style that isn't real heavy but will be warm enough for most winter days, even in the cold climate where I live.

If you find a pair you like except for the length, take them to a local alterations shop to see if they can be shortened. Take them in before removing tags in case they can't do it. It might depend on the fabric and style, but it wouldn't hurt to ask.