I don't grow oats, but if I wanted to try it on a small scale I would probably buy a little packet from some place that sells garden seeds. Many of the online seed companies have oats, typically in the "cover crops" or "seeds for sprouting" categories.
If they grow well, you can harvest all the seeds that year and replant it for a larger patch the next year. If they don't grow well, you haven't wasted a large amount of seed or money or space.
Or if you buy whole oats to feed any kind of livestock, you could try planting some of them. Whole grains generally should be able to grow (although rolled oats would not grow, of course. You do need them un-crushed.)
Both of those ideas are an easy way to try a small amount, but there's a good chance you won't have the "best" variety for your climate.
If you want to grow entire fields of it, or you want to have a higher chance of good results, I agree with
@U_Stormcrow about who to ask. They should have some idea of what varieties grow well in your area, and where to buy the seeds.
