Basically, any ZigBee or (especially) Z-Wave device should work with Hubitat. You can check the the official and community list of supported devices, or search the forum (or ask) if you're not sure. A lot of LAN devices also work, but generally only ones with an open API like Philips Hue (if you use the Bridge; ZigBee bulbs can also work directly with the hub, though I wouldn't always recommend that), Sonos, and the like.
Personally, my favorite motion sensors are the Iris v2 motion sensors, though others have reported the new v3 Iris sensor also works well (and the v2 has probably been discontinued and will only be harder to find). The "new"/2018 SmartThings Motion Sensor also works well for me. It's bigger but does have a mount--Iris has tape tape or needs a "set on a ledge"-type (or "make your own mount") situation.
For in-wall switches, you have a lot of options. Inovelli is generally well-reviewed but currently mostly out of stock. ZooZ is similar and may work well, plus there are a few options from HomeSeer and lots from GE (make sure you get their Z-Wave Plus models, not "regular" Z-Wave, if you go that route). I have a HomeSeer switch becacuase I like the LED and wanted multi-tap for different "scenes," the latter of which Inovelli and ZooZ also support (and you won't need an expensive add-on/companion switch for the second location in a three-way with those). If you think you'll have a ton of switches or if this just sounds better to you for whatever reason, you might also look into Lutron Caséta with the Pro Bridge, which will give you that plus the ability to use Pico remotes (with or without hardwired switches) anywhere--Hubitat's integration is really neat.
Most switches are Z-Wave, but there are a few ZigBee (GE makes one, I know) available. There are both Z-Wave and ZigBee motion sensors, but I think most ZigBee sensors work better (or at least they're faster, and I'm using most of mine for lights, so it matters). If you have a large house or devices far away from the hub, you may need to plan to have powered/repeater devices between the hub and your "end" (battery-powered) devices, which I mention only because ZigBee and Z-Wave form their own mesh networks and may need "extending" if you're far away (or in the case of ZigBee have >32 devices). Some people like to stick to one protocol; many, myself, use a mix of both, depending on the device. Hubitat supports and ties them all together so it doesn't really matter much beyond that.
As for where to buy: besides the Iris products at Lowe's (and non-Iris products at similar hardware stores), you can find a lot of these devices on Amazon, TheSmartestHouse, and similar sites, often also directly from the manufactuer's/seller's site (HomeSeer, Inovelli, etc.).
I wonder if Hubitat wouldn't benefit from selling (or suggesting) a "starter kit" like another platform does to make this easier for brand new users.