First of all, say where you are from. You don't want to be ordering from the other side of the world.
You won't find anyone selling thousands of them, but you will find smaller amounts which will be plenty for you. If you have a bug infestation you want to control, inside or out, just set them free on your plants and they will do their work.
If there is enough bugs to keep them living then they will eat through a fair amount of bugs. The adults can lay up to 30-50 (if i recall correctly) eggs a day and will reproduce to keep the chain going.
If there is not enough bugs to keep them alive, well that's a good thing. Problem sorted, just keep it from happening again.
A quick Google search for ladybugs along with your region of the world will find the nearest breeder/seller of ladybugs. Most are wild caught, so the quantities are not huge. Only certain types are captive farmed.
You are not supposed to release wild caught into the wild in your area due to the risk of spreading parasites that are not already in your region.