Beech has a lot of issues with checking and movement when drying. Given marketability in the US, it usually isn't profitable to cut, steam, and kiln dry thicker (>8/4) stock. The same checking issues show up when air-drying. Unless there is a specific buyer it usually isn't cut more than 10/4.
In the past I've had good results sourcing beech from folks who saw urban-grown trees, but that often means waiting until they have a beech. Another source has been sawmills providing wood to the pallet-makers, though that can mean sorting through a semi-load of wood to find what will work for you. Once in a while one of the lumber suppliers to the cabinet trade will have some on hand (I once got a very nice piece of Euro beech from Paxton: Q-sawn 12/4 S2S, 10"x8'). If I wanted it today I'd reach out to the folks supplying the people who are making wooden planes and ask for contact info for their sawmill.
I've got some 12/4 beech I sawed and stacked in the barn loft in 2005 or so, but I haven't looked at it since then. Don't recall if any pieces would be suitable for stocks.