What's your price point and spec desire/use case? ARM or x86-64?

There are good ARM SBC alternatives with good community by ODroid and a few other brands for $30-$100. ODroid probably has the biggest and best community outside RPi.

Used thin and zero clients can be had for very cheap and used for low power, low resource needs.

Alternatively, the new ultra low power Jasper Lake CPUs are pretty insane for the price/power and you can get complete mini PCs with "real" nvme or sata storage, dual 4k@60Hz video output, 1 or 2.5Gbit Ethernet for $150-250. Search for n5095 (15W TDP), n5100 (6W), n5105 (10W), n6000 (6W), or n6105 (10 W) mini PC on Amazon. The iGPUs on these are killer and can handle 2-3 4k transcodes too. They make great media/storage servers (that's what I'm using one I just bought for)

I love SBCs, homelabbing, and squeezing as much out of hardware per watt and dollar. Liliputing and ServetheHome have been great resources.

https://liliputing.com/tag/mini-pc/

https://www.servethehome.com/

If you just want a little Linux box and don't need GPIO, I'd recommend buying a used "thin client" from eBay. These are usually low-end x86(-64) machines with decent connectivity but not much storage, as they're meant to just display the UI for apps running on a server somewhere. You can generally hack more storage into them pretty easily.

A 4-thread machine with case, heat sink, power supply, 8gb ram, and an SSD can be had for less than the MSRP of a bare 8GB RPi 4.

One downside is power: the thin clients generally idle around 5-10W and use 20W or more at peak; compared to the Pi which will idle at 2 watts and use 10W peak.

http://parkytowers.me.uk/thin/ is a great resource for understanding the different models that are available.

I've had some success using this page and subscribing to the twitter feed from the same person: https://rpilocator.com

Adafruit appears to be the best resource for ordering Pis in the US as they require you log in with MFA and limit purchases. That is, if you're just in the market for a single unit.

oDroid units have been OK for me.