I would start with Craigslist, and Facebook Marketplace, those seem to be the go-tos for ads these days. Also put an ad in the wanted section of your local farming newspaper, if there is one.
Thinking about a custom build trailer. The rear end would be a tow behind stumper like a 630A but the front half of the trailer would be a sideways mounted dump box. So the whole unit is self contained and towable
I found a couple on CL that I’d jump at but they’re in the US and I’m in Ontario so I’m not essential to cross the border to go check them out.
You guys have some good deals down there