Installing apps from Android Market or alternatives to a virtual Android x86 device I'm using Oracle Virtual Box to emulate an Android-x86 device. I installed android-x86-2.2-r2-sparta.iso to a virtual HD.

Are there any way to install apk packages from Android Market (http://market.android.com) or do I have to download them only from "unnoficial ways"? Any suggestions for the 2nd case?

PS: Note that I'm asking for a solution for that particular case.
I am not familiar with that particular distribution.  However, in general unless the build you installed contains the "Google Experience" apps (or if there is a separate Market package for it,) you will not be able to use the official Android Market front-end to send apps to your virtual device.

There are some alternate markets, such as Amazon Appstore, GetJar, SlideMe, etc.  Most will require you to download and install their own Market app in order to get other apps.  

If you already have APKs of the apps, you should be able to side-load them either via ADB shell (from Android SDK), or via the bundled package installer by executing the APK from the storage area with some sort of a file system browser (if included with the build.)