Sometimes the directions below will fix the problem:
Try restarting your phone, and connecting via wifi. Also try:
1. Open Settings.
2. Select Applications.
3. Select Manage Applications.
4. Select Market (You may have to press Menu > Filter > All to see it)
5. Select Clear cache.
6. Back out of the screen and select Download Manager.
7. Select Clear data.
8. Also clear data for Google Apps, Google Talk, and Checkin Service.
As far as other places to download apps, AndroidPit has it's own market: download the App Center here http://www.androidpit.com/en/android/market/apps/app/de.androidpit.appcenter/App-Center
Click the AndroidPit download link.