The RISC-V ecosystem might still be a nascent one, but it's definitely starting to take shape. You can now order the Milk-V Titan full-featured Mini-ITX motherboard kit with an integrated Ultra-RISC UR-DP1000 CPU (RISC-V), all with standard hardware, and ready to roll.
Although this isn't strictly the first such offering, it's one of the few on the market that combines complete feature, out-of-the-box usability, and a reasonable price. The motherboard is a pretty plain Mini-ITX model, but in a good way. It supports up to 64 GB of DDR4 RAM in a dual-channel setup at up to 3200 MT/s, and has one M.2 slot, USB-A and USB-C ports, Gigabit Ethernet, and BMC (out-of-band management) ports.
The only notable omission is integrated graphics, as you'll have to make use of the available PCIe x16 slot to plug in your own graphics card. As RISC-V is for practical purposes an entirely new platform, graphics driver support is still somewhat spotty. Older Radeons (7000 series and previous) are known to work well, but this very statement is likely to change quite quickly.
There are no audio ports on the board, but that's unlikely to be a deal-breaker as you can always use USB audio devices. Besides, these boards are aimed at development work anyway. In fact, there are even 3-pin UART and USB-C connector for CPU debugging purposes. The idle power consumption is apparently pretty high at 14 W, but that's not likely to matter for development purposes.
As for the Ultra-RISC UR-DP1000 CPU itself, it's an eight-core setup with four two-cluster cores, each loaded with 4 MB of L3 cache, for a total of 16 MB. It is fully compliant with the RVA22 profile (RVA specs are CPU instruction sets), and there's support for the RVA23 except for the V (vector) extension. It's important to note this CPU supports hardware virtualization, so you can use hypervisors with it. And, of course, at only 2 GHz on a nascent platform, keep your performance expectations tempered.
You can run Ubuntu on the The Milk-V Titan right out of the box. The kit available is for preorder now at Arace Tech. The standard price is $329 or 288€, but there's a $50 discount for preorders, so make that $279 in practice, a pretty reasonable amount. And since it uses DDR4 RAM, you might be able to get the memory for less than the entire board.