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.
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.