It depends on what you are targeting. New/used, truck, chipper, local/regional/domestic/international, etc.
You can set up rss feeds for craigslist with tags for what you want.
You can search Facebook marketplace.
You can join a Facebook group for an item/category you want.
You can cold call rental stores for their used units.
You can become listed with an auction company or three.
The only limiting factor is your ingenuity, which is often driven by how bad you want something.