Stock photos are all over the internet. Try Shutterstock.com, or 123rf.com and others. Licenses often vary according to use as well as size, so read carefully. You can also do a search for 'free stock photography' or other terms, but, as a rule, photos there are not as high quality as pay sites.
Be specific when you ask for hi-res photos. Let your customer know the file size you want as well as the resolution and type (.jpg, .tif, raw, etc).
If you'd like a couple hi-res photos to practice with, let me know what subjects you prefer and what size files you like, and then I'll send you a couple. I shoot in Nikon's raw format then convert to whatever format is necessary for use. Doing it this way keeps a lot of details lost in the internal conversion process.