That's true - you can find anything you want, and the file sizes are so trivial, right? The problem is that you're explicitly working outside the chain of trust. You have no earthly idea what's being installed along with those .APK files, whether they come from something that purports to be an app store or a torrent or just some random dude's Skydrive account. Safe sex analogies work really well here, and what you're doing by installing indiscriminately is pretty much exactly as safe as getting boned by an anonymous bisexual, heroin-addicted Haitian rentboy with open sores about the perineum, sans prophylactic.
There are very few mobile games or apps that cost more than $4. Once you have some kind of income, that's pretty trivial. For now, you're probably willing to tolerate internet herpes if you can play Plants vs. Zombies for free, but this is a problem that will eventually fix itself, either by you getting a job or by you getting internet HIV and having to re-set your tablet.