I often order clothing online. I always make sure I can get free shipping (Google "free shipping for ___") and then also make sure I can make a return to a nearby store. I never pay for shipping but end up returning lots of things. However, I just found out about sizewize.com, which is a website where one can enter measurements and select from 150 brands of clothing. Apparently it will suggest the size you need in various brands. Unfortunately, it doesn't appear that one can enter inseam measurements. My biggest problem with clothing is usually the length.
Also, try styleforfree.com. They have lots of discount codes if one was planning to order something from a particular retailer.