With a little advance planning, you can avoid ever having to purchase overpriced travel-sized items. Here's how I do it.
Companies are constantly giving away free samples of new products. I request free samples of everything I think I might use. It usually takes 6-8 weeks to receive the freebies, but I usually have a constant stream of them coming in, which means that I always have enough shampoo, conditioner, and lotion for traveling.
Usually, these items come in small flat plastic packages, not bottles. I didn't have any trouble getting through airport security with these in my carryon (just put them in your quart ziplock bag with your other toiletries).
Another thing I do is use old film canisters to store items like lotion and face cream. I haven't tried taking these through security in a carryon, though. With film canisters becoming rarer these days, your best source may be to simply ask for a few from the photo department the next time you're at the drugstore. The canisters are airtight and leakproof, and hold a surprising amount of product.
Also, I often take hotel shampoos and conditioners with me when I stay at hotels. I hate the stinky shampoo they provide, but I dump it out and save the bottles to fill with my own products. Whenever I acquire a travel-sized bottle, I tend to hold on to it and refill it as needed.