OK, so if it's a basic braided challah, I usually just buy it. Platting is not my thing.
Now, if it's for Rosh Hashanah, it should be homemade. It's not that you can't buy one, but your Rosh Hashanah challah should be round. There's a superstition that the rounder your homemade Rosh Hashanah challah turns out, the better (i.e. well rounded) your year will be). I cheat if I'm going to do the whole High Holy Days things. There's a brand that sells frozen raw braided challahs. I usually buy two, let them thaw and rise once before I try and form them into something round before a second rise and baking.