wait for them to show up at your nooks, or catalog them from someone else (drop trade, when you pick the item up and drop it, it'll register in your catalog and you can order it from there)
One color should show up in your Nook's eventually. The other seven can be obtained via Redd, but he almost never comes, so I wouldn't count on that. If you don't really care about the color, you just have to be patient. If you do care, there's a fair chance you're going to have to trade with others for it.
(Also, if you need to catalog all of the simple panels colors, let me know. I can do it for 30 TBT.)
also got my white simple panel from a balloon. the only other variant i've seen is the silver one from nook's. definitely keep your eyes on both of them!