You can either go to your doctor and get a script (very easy, I've done it for years) or buy online from a pharmacy. Here they are
1: Megace (generic Megastrol) -- Liquid
* Side Effects: Uncommon. For me, zero.
* Effectiveness: 7.5/10
* I take this in the morning, and hunger persists throughout day. When taken after a meal instead of in the morning, it makes you hungry far faster about 1.5 hours after. By very hungry, I mean you could eat 1500+ calories at a buffet. The taste of it is BAD but luckily is gone within 3 seconds of taking it!!!
2: Periactin -- Pill
* Side Effects: Drowsiness
* Effectiveness: 9/10
* Most recommend you take this at night, due to it making you drowsy. If you take it at night, I have found it doesn't make you as hungry the next day. If you take it during the course of the day, I have noticed it being more effective. You never really get full when you take this. You could eat 4000+ calories a day without any trouble.
Just a note, I have a very weak appetite normally. I used to only eat 1-2 meals a day in high school a few years ago and not think about it. So the fact that these two increase my appetite that much means someone with a more fairly normal appetite will probably never stop eating... so of course make sure you are eating healthy