Where can I find them, and if I can’t, then how should I train my mechanics skill to 8?
you can find them in garages, public works buildings, I think hardware stores…that’s all mainline, right? In the interim, you can make makeshift welder n put UPS mod on it. Vehicle ups recharging rigs are easier to find iirc.
It is also craftable. According to the item browser it requires electronics 4 and a book with the recipe.
Do note you’ll need another welder to actually install it to the vehicle I think.
Construction sites also have a chance of spawning welding carts which are basically vehicle welding rigs on a frame.
But never, ever try to make it using 3 makeshift welders. You have been warned.
Junkyards will also spawn welding rigs.
In my current game, I found one in a Junkyard along with an engine crane (a telescopic crane on a frame).