ASRock's B450 motherboards are (supposedly) now shipping with a compatible BIOS and a "Ryzen 3000 Ready" sticker on the box. But you'll probably need to buy from a brick-and-mortar store, so you can inspect the motherboard before you buy it. That's essentially impossible with an on-line vendor.
MSI Launches New AM4 Motherboards with Ryzen 3000 Support MSI is launching a new line of socket AM4 “Max” motherboards using the AMD A320, B450, and X470
ASRock's B450 motherboards are (supposedly) now shipping with a compatible BIOS and a "Ryzen 3000 Ready" sticker on the box. But you'll probably need to buy from a brick-and-mortar store, so you can inspect the motherboard before you buy it. That's essentially impossible with an on-line vendor.