Whenever I perform rectal palpation, I can never get my hands on uterine horns, but ovaries are palpated well by indirect approach. How can I find the uterine horns?

This isn't going to be very helpful, but you won't be able to find them. The uterine horns are going to be very difficult to find, because they are as soft tissue as the rectal wall. What you're really going to be feeling for is the more harder, denser tissues like the ovaries, the cervix, and the small embryo-turned-fetus. You're doing everything right if you can feel the ovaries and, if you went more shallow, the cervix. The fetus can be felt in the uterine horns, so that's what you're really going to be feeling for, not the uterine horns themselves.