What is the most cost-effective place to buy large solar electric panels?

We don’t have enough money right now to buy capacity for the whole house, which would probably require about 12 kilowatts. However, I would like to start buying panels toward building up toward that.

But I want to buy the most solar panel for the least price. I think that piecing a lot of very small panels together will be a very expensive way to do it.

Does anyone know the least expensive way to buy large solar electric panels?