I believe most 3D printing models are organized through Blender. I'm not one, so I'm not sure. It's what I see usually being recommended. Some of the people who do so post their builds on sites like Thingiverse for 3D printers to make, or if they have a printer, sell through sites like Etsy.
Ironwind Metals (and a few online retailers of the same like Aries Minis and Fortress Minis) also sells bits which can be used to modify a Mech in to a desired design.
I did this with a Zeus-X I'm planning on modifying for a biweekly campaign. I took the missile launcher from a spare arm I ordered, cut off the barrel of the autocannon on the Left Arm, and then pinned/glued it to where the autocannon barrel was. Looks pretty decent.
Images from the metal work: