You can make up your own from separate seed mixes.Sorry, I still can't get the hang of this reply stuff! What mix of grasses and broad-leaved greens and weeds do you use?
But I buy bags of premixed seeds.
Look for 'Chicken Forage Mix', for example.
Here are some examples
https://www.groworganic.com/omega-3-chicken-forage-blend-irrigated.html
https://hancockseed.com/hancocks-happy-hen-forage-seed-mix-5-lb-bag-976.html
I let them grow for 3 months, then lift up the grazing frame and let the chickens have at it. Once it is bare dirt, I add an inch or two of soil, reseed and replace the grazing frame.
Good luck!