Paint or color-by-numbers puzzles are available for free online on websites such as allkidsnetwork.com and hellokids.com. These sites offer free, printable illustrations with numbered, color-coded sections.
Hellokids.com offers a vast variety of options. There are more than 90 different printable pages that are free. These puzzles are easy to find online and offer a cheap alternative to paint-by-numbers coloring books, which are sold in most arts and crafts stores.
A disadvantage to printing them from an online source is the lack of variety; you may have to settle for images you don't like as much as others you might find in a store, but if you are simply looking for any kind of paint-by-number activity, the Internet is as good a place as any to narrow down your search.
For a slightly different paint-by-numbers experience, adults can check out segmation.com for a grown-up, digital version of the children's activity. This website advertises "the art of pieceful imaging" and proves that painting by numbers is great for adults as well, particularly those with an appreciation for digital art and design. There are dozens of options to choose from, including many classic paintings with a lot of challenging detail — and signing up is free.