Assuming you meant you just cannot find anywhere, it depends - if it is a paid app only, then don't bother asking anywhere in the forums - the only answer you'll get is the developer's personal website and / or Market.
If you meant an app for your computer, basically the same applies - if it is shareware / commercial, your request comes across looking as if you're requesting illegal pirated material, and you'll be (probably) banned in an instant.
if you're looking for an old, no longer developed piece of software for your compute, here is the best place to start, I'd reckon.