You can get the .apk files from developers' sites or other forums like androidfreeware.org and alternative markets like slideme.org.
Al Everett's answer above is part of the solution.
After changing the settings to enable you to install 3rd party apps, you can either upload / add the APKs of apps that you want on to your SD card and then using a File Manager, install them by tapping them and letting the Package Installer run the setup.
OR
You can navigate the sites listed above and download the APK using the default browser - I recommend you install a file manager like Astro so you can navigate to your SD card and install apps.