Your approach is a good one. Do you know anything about the size of the cock you're inviting?
The recommendations so far lead you to pretty extreme butt plugs. They come in small (not much bigger than a finger), medium, which will be a bit of an experience to take in, and large, much bigger than the average cock. And then there are a variety that are simply huge, like some cocks. The aneros thing, by the say, will not help you stretch your ass, as you wisely want to do, but is to stimulate your prostate. But that's another story.
You can find the ordinary butt plugs on the net easily, or in "adult stores." Unless you expect your ass to be meeting up with a very significant cock, that's likely all you need. Lots of lube is essential as you experiment, and a condom over the plug makes clean up really easy.
Have a great time!