The UGREEN NAS series can readily run pretty much any OS you like, including TrueNAS. The DXP2800 is a two-bay unit for $349. But you’d probably want to use one of the NVMe slots for the OS installation.
It’d be a pretty low-end system–you wouldn’t want to plan on running VMs or much in the way of apps on it–but from what I’ve seen, they’re about the best going in terms of a pre-made NAS for TrueNAS.