Backgrounds are usually drawn and painted with watercolor or gouache paint. Acrylic is not normally used for backgrounds because it has a nasty habit of sticking to itself and thus might stick to and damage the paint on cels. Backgrounds are a specialized field of animation. They require a lot of talent and a good understanding of blocking. Even though modern day animation has switched to digital many studios still utilize hand painted backgrounds. While some can be pricey they generally will not cost as much as you would think.
Original Background from Saiyuki Reload: Burial OVA Episode 1, Sanzo's Chapter