There are some services which will send you a list of subscribers along with their full names, I.P. addresses and date/time stamp. While having all of these as proof that someone opted into a list, it doesn't mean that they opted into your list.
If you buy a list of these leads and email them and get a Spam complaint, which will surely happen, you can stay out of trouble because you have all the information as proof that they opted in but it's a continuous fight.
Still, I wouldn't go this route. I once bought a list of 10,000 names and after trying to show them that I'm a nice guy and want to help them, I didn't get much of a response from the list.
I gave them lots of free stuff but they weren't interested.
Pay per lead programs.
These programs are a little better than the above method in that they list your optin form along many other forms, and people have to check your form and fill in their information to get placed directly on your list.
Let's say you pay for 500 subscribers for $50. You pay that money first and then they will list your form on one of their partner sites until you get 500 subscribers or the number that you ordered.
The better the description you create, the faster you will get to that number that you paid for.
The bad thing about this form of optin, which I've also tried, is that people see a bunch of checkboxes, and they just click through each checkbox impulsively. That means that they'll be getting emails from many other people all at the same time, and then they end up getting overwhelmed and unsubscribe from all lists. Heck, I've done this myself.
Those are the two methods of list building which I've tried and will not try again because they produced poor results for me and will do the same for you.
You might get different results, but if you don't want to risk being disappointed, rather try some more successful methods to do so.
