Are there any widely-used sources or sites for Transformers weapons? I've seen people post pictures of Generations Springer with a gatling gun and I know about Dr. Wu, but where else have people been getting their aftermarket weapons? I've tried Google and this site's search function; I'm just not having any luck.
I'm not sure about website specific but I do know a lot of people on this website sell individual weapons like Sportinmorton.
Pre-AoE construct-bot kits are good, cheap sources of 5mm compatible weaponry. Thundercracker comes with a pair of cool assault pistols. The standalone Megatron as well as Dead End come with some nice looking swords.
Also some of these are could be weapons made for Gundam model kits. Anything from guns to swords and whatnot. Just look for MSG Weapon units or FrameArms. Cost is not too bad and you do have to put them together. And the swords can be not a perfect fit in the hands but a little bit of foam can fix that.
2 sites come to mind. transformerland.com shapeways.com (I know this is a primary source for 3D printed weapons) You could also try a "Transformes weapons lot" search on eBay, but most sellers list 1 weapon as a LOT, so I use that as a last resort.