I bought some off Amazon a couple of years ago. They work okay, but have a slightly different thickness. So if you go this route, either go all in, or screen carefully if you are going to mix and match. It's been a bit, so maybe they've improved.

By the way, I believe road plates are the only thing so far that's been phased out. I still see baseplates on sell at Amazon and Target all the time.

LEGO Classic Blue Baseplate 10714 Building Kit (1 Piece) https://www.amazon.com/dp/B075QRYDFB/ref=cm_sw_r_apan_glt_fabc_NSVH0BWF05ZS81SXM442