The ST button mentioned above is cheap and would be my recommendation if you only want one or two. But really, you can use any button device Hubitat supports. There are many, and they've summarized the ones with built-in drivers (and the capabilities they support) here: Hubitat Elevation supported button controller capabilities matrix
If you feel like buting super-adventerous, the new Eria ZigBee Dimmer cheaper than the Hue dimmers (which can be paired to Hubitat and may work better for you if you want dimming buttons or extra buttons for any purpose). However, it's new and isn't officially supported. You might get some functionality with an existing driver, but if you've ever wanted to modify or write a driver, it would be a good learning opportunity. 
(If it's doesn't use ZigBee group broadcasting or something Hubitat doesn't support in its present state, I'm sure we'll eventually see a native driver, and if any existing driver works they could easily add support if you provide the fingerprint and whatnot you get when pairing.)
For me: because I decided I really like buttons, I got a Lutron Smart Bridge Pro and am using Pico remotes for most of my new buttons. They're cheap, but the Smart Bridge Pro isn't, so you'll need to buy a few before you break even (but not really that many: 6 Hue dimmers * $24 = $144, while 6 Pico Remotes * $12 + 1 Pro bridge @ $80 = $152). But they're lovely because they can mount just like a standard switch in a decorator plate. Just something else to consider depending on your needs!
