Your utility may not supply the meter socket, as most don't, but most utilities list the make and model numbers of the meter sockets they approve and allow.
The local electrical supply distributor is probably the only place you will find a 320 amp socket, home centers would not,since this is a bit above the level of your average DIY'er. And the local distributor will only carry the sockets approved by the local utility, but check with the utilities requirements first.
Most utilities have their service requirements available at their website, but some may be hard to find or recognize, let me know the utility and I can look for it.
I included a picture of a 320 amp socket from Milbank
, a very popular manufacturer. Note in their catalog, the different sections of the country each has a different group of meters. The 320 amp socket does not come with any load lugs, they offer various load lugs, you would need each leg to have a double lug for two 200 amp wires.
The 400 amp main can be either, for each leg 1- #600 XHHW aluminum, or each leg can be 2-#4/0 XHHW Al, or 1-#400 MCM THWN copper or 2-#2/0 THWN copper.
Each leg to a 200 amp panel can be either 1-#2/0 Cu or 1-#4/0 Al.