Linda yes the curved ones are
sold separately but if you already have wheel chocks you can use them instead - just put an extra block on the top row so it sits flat.
It unfortunately not self leveling - dont think any of the options are. I have small stick on levels you get at rv stores - one under the belly band on the side of the trailer that gives me a reading of the front to back level and one on the front of the trailer for the side to side level. Also have a small bulls eye level on flat part of the tongue of the trailer that gives me a pretty good idea as to where its all at before I take the tongue off the tow.
All I do is put the trailer back about a foot or so from where I want to camp take a look at how level it is side to side and lay down the blocks in front of the tire on the low side and then pull the trailer up onto the blocks. Once you have used them a couple of times its pretty easy to tell how many levels you will need to get it level ... even if you get it wrong its not a big deal to back up a foot pop another block on and pull forward again.
I also often use the blocks stacked under the stabilizer jacks at the rear as well - if I used blocks under one of the
tires ts a good bet the low side stabilizer isnt going to touch the ground either. Also come in hand to use stacked under the tongue jack in situations where the land sloops away sharply making it impossible to jack the tongue up high enough to clear the hitch ball.