Alternative to App store

Have just got my first iPad and generally am very pleased with it. Except for one thing.

An app I want is no longer available from the app store. So my question is - is there any alternative to buying apps or do I have to use the Apple app,store. And if the answer is yes, I have to use this method, is there any legal issues regarding anti-trust and market dominance.

Thanks.

As far as I know, the only source for apps for Apple products is iTunes. I have not heard of any legal issues coming from that requirement.