Transmogrification is the best thing ever—second only to armor dyes, which WoW doesn't have.
In lieu of dyes, we can search for things to transmogrify our things into and out of. But to do that requires an unspeakably vast knowledge of the gear in WoW—or a handy-dandy site to help you find a medium-length purple cloak with a gold border.
I found some!
This was the first one I found and it still remains my favorite go-to place. Good, simple, interface that gets the job done. It has a great index of everything I've looked for in terms of clothing, and more. Plus the creator is on Twitter and is very nice!
Next up is a good site for displaying sets of clothes. I perused this site quite a bit, just going on a general tour looking at what all's available and looking for ideas. It was the first place I saw an occupation category and that's pretty clever. It's a good guide, and a good source for ideas.
This site doesn't have a grand guide to all possible items but instead profiles a specific outfit and tells you how to go about getting those specific pieces. I like this site because it's nice to see what people are doing when they mix and match various items to create their own look.
This site has a wonderful resource: A guide to all weapons. If you're looking for the perfect staff to go with your outfit, or sword, or whatnot, you simply must go here. For example, take a look at this image:
Pretty cool, right? And on the site are instructions for how to get to each one. I also love how they're organized according to how many similar models there are of each item. There are lots of staves like those in the lower right, but there's only one specific staff for those on the upper tier. Handy if you want to ensure your character's look is rare or unique.
Last—but certainly not least—is Wowhead's tool that is quite keen. It begins with a basic set of items then you can filter that selection to quickly find just what you need. A very handy starting off point and it's easy to use. You definitely want to give it a whirl.