Last year I grew mine in a raised bed and they were covered with Enviromesh
I grew my first ever Tatsoi in a big tub that I'd used for growing spuds. I used the same compost, just planted them straight in, and they did very well. I left them to develop into proper rosettes and then ate them, rather than just individual leaves, but that would be up to you. I think flea beetle might be a problem too but I planted them late in the season so maybe they were not around then. Enviromesh is really good stuff and keeps bugs out as well
as pigeons etc off the crop
You can get the seed anywhere as it's very common