www.cgtextures.com - on the left side menu you will see a subject heading of "Water".
In that set are waterfalls, as well as foamy water, etc.
Unless you have an account, there is a 15MB download limit per day (every 24hrs). This is not a big deal - you can actually download a significant amount of images per day - they are jpegs with a decent quality to filesize compression.

Check out stock photo sites - some are pretty cheap if it is for personal useage. Try www.istockphoto.com for cheap rates and decent sized images. You can find a good picture of a waterfall, mossy rocks you can use, etc.

Just for giggles, do you have the photo you are working on, and are you in a position to upload it here (this is a good portion of what I do for a living, so I might be able to throw some other thoughts your way if I can see it)?

When it comes to making it look like dripping water on her skin, there is a great Layer Style made by Burt Monroy (a PS genius!) that for the life of me I can't remember right now. Looks awesome - VERY realistic and pathetically simple to paint onto a picture.

Hope this helps-
-Lew ;-)