Have never bought ladybugs, but read about them. Have always had enough hanging around that I didn't have to buy any. Best time to release is after dark. They will still fly away, but more will stay than if released during day.
No self respecting aphid, mite, white fly or any other bug that lady bugs eat (or their look-alikes, Asian Beetles) would come within miles of our area, and if they do, they are summarily eaten by the millions, no billions of lady bugs we get here in Northeast, PA.
Each Fall, after the corn crops are cut down, we are inundated with Lady Bugs throughout our house. The problem gets so bad, we actually have to vacuum ceilings, floors and around windows sometimes twice a day. It seems the Lady Bugs are looking for a warm place to winter. They will get in through any nook or cranny, and are especially attracted to light color houses. So with an old farm house that is white, we get bombarded. They then winter in the walls and in the Spring it seems there are twice as many. It really is a problem in this area, and many other rural areas in the country.
After the first few years in our house, we decided to get insect bombs. We set them off in the cellar and attic, and went out for the afternoon. It helped, but we have to perform this extermination every couple of years, because vacuuming alone only gets a small percentage. I don't like the idea of using a pesticide in my house at all, but I am tired of constantly vacuuming, accidentally drinking, sitting on, stepping on and swatting away Lady Bugs in my own house.
We have that too, @MoonShadows. Ours mostly collect on windowsills and I just sweep them off occasionally. There are still aphids in the garden though.