Ok, the Avigo appears to be what we here call a "Bicycle Shaped Object" (BSO). The bike has front and rear suspension and retailed for ... $99. This is about the bottom of the barrel, quality-wise. There will be no replacement parts available. You can get help here, but be prepared for the case where you fix one thing, and another breaks.

Right now I'm not sure if the bolt you mean is the bolt for the derailleur hanger, or the one that goes through the derailleur body and screws into the hanger. The former is likely a standard sized bolt that will be readily avialble. The latter is larger and more specialized and I don't think you'll find it. So you may have to replace the RD.

Velotec does not appear to be a part that Shimano makes available as a replacement.

Take pics, post to google pics or imgur or another free fileserving site, and post the URL link to the pics. This will help zero in on your problem.