Woah... way too much money for WAY too many pills!
You can buy aloe vera pills in the herbal section of any pharmacy (Walgreen's, Wal-Mart, etc.). A bottle of about 100 should cost you around $6-$8, best I recall. The bottle will probably have a suggestion as to how many to take a day for health, but since you're taking it for constipation, you'll have to just figure out through trial and error how many you need to take a day.
I don't recommend the juice because there's two types and only one type works for constipation, and everyone agrees that it's thick and slimy and just plain nasty. Why drink swill when you can take a pill?
My mother started taking them for hair loss some years back. I think she was taking the recommended 2 a day, but it was making her stools unformed, as if she had taken a chemical laxative. She had to cut back to one every-other-day in order to maintain regular bowel function. She didn't have any prior bowel problems.
I would suggest between 1 and 3 a day. If you are only mildly constipated or only constipated some of the time, start with one every day. If you are pretty constipated most of the time, try three. If you start to get too loose, reduce your consumption by a pill, or take one every other day if you already take only one; if it helps, but not enough, then try taking more than three a day. It may even be something that you can't be consistent about. You may need to take a pill a day until you get constipated again, then take two a day until you a little too loose, then go back to one, or something like that. As far as I am aware, aloe vera is not habit-forming, you can't overdose on it, and there are no side affects except laxative effect (which is what you want).