I grow my own, and it grows wild like a weed here too. It doesn't have to bloom, I just take leaves off whenever during the growing season, and at the end of the year I dry some.

I originally just bought one packet of seeds and started 3 plants for setting out - 3 plants is a lot of catnip.

But if you aren't the gardening type, buying it is the best thing - among brands, I don't think I know of a difference. I'd try and by organically grown, to cut back on any chemicals growers might use - but in my experience catnip plants aren't bothered by bugs.