It only takes a few minutes to make a barrel in Sketchup. Create a cylinder. Duplicate it and scale down the duplicate a little. Then boolean subtract it from the first barrel to hollow it out. For the ribs, create a torus and size it properly. Duplicate it several times and position them where you want.
You either need to learn a program like Sketchup, which is easy to learn, or find existing model files for download.
While certainly Steve’s method works, not at all how I did it. I did mine is SketchUp. I drew a profile of the barrel, and used the Follow-Me tool to follow a circle the diameter of the barrel. And, there are several other ways to draw it.
There are a few web sites where one can get printable items, but very few are model railroad related. I have never done any scanning, but from what I have heard, there is still a lot of clean up that must be done in a 3d drawing program. There are lots of items in the 3d Wareshouse that is part of SketchUp, but I have never seen an item in the warehouse that was 3d printable. They all take time to edit, or in most cases, it is just about as easy for me to start from scratch.
I was not able to print HO scale concrete blocks on my Afinia printer, as the software would not allow walls thin enough. The M3D will allow me to print walls down to at least 1-1/4 scale inches in thickness. For more pictures and design details, click on this link, http://nvrr49.blogspot.com/2015/10/m3d-printer-3-concrete-blocks-in-ho.html