Did I just made an overview with SP3 motherboards? Yep, I sure did.
But...where can you buy them?
Well, say you live in the UK, you can go to e.g. Scan Computers.
They offer four SP3 boards, two single socket Gigabyte ones and two dual-socket Supermicro ones.
If you live in the USA there is Newegg -yup, from the banner above- that even stocks nine (eleven counting the out-of-stocks) SP3 boards -ten from Supermicro and one from Gigabyte.
Seven of the single socket Supermicro boards already have an EPYC installed -making prices soar up from $756.99 for an 8-core EPYC 7251/Supermicro MBDH11SSL-I combo to $5,051.99 for a 64-core EPYC 7702P MBDH11SSL-NC combo. Even one of their three dual socket mobos already is filled with CPUs: the Supermicro MBDH11DSI comes with two 24-core EPYC 7401's for $3,079.99

You can of course also contact the local agent of e.g. Gigabyte. Gigabyte Germany has no less than six EPYC 7001 mobos -and a EPYC 7002 mobo, bo so does Gigabyte Netherlands. There's no doubt that Tyan offers a likewise portfolio.

We already saw that Newegg has more-or-less affordable EPYC/mobo combos for sale, they have CPUs too and the same holds for Scan Computers and Alternate.de