Go to IMVU's catalog and do a search for the species you're looking for (for example, "wolf" or "fox"). When the search results come up, you can narrow the results down to just skins, or just accessories (which would be ears, tails, wings, etc.) or whatever you want. Once you've found something you like, you buy it using your credits or promo credits, and put it on.
I make furry items myself, and you can find a link to the things I've made in the catalog link in my signature below. (Not saying that you have to buy from me or anything, just thought I'd mention it in case it helps.)
