Many companies (more than you would expect) still produce and sell paint-by-number kits. Unfortunately, many that you see in craft stores are for children, which are not only limited in complexity but also in subject matter. There are far too many adorable baby animals and sea creatures available. Far too many.
However, the best quality kits are still available online and the best are landscapes, some of them vintage designs like mine below.
There is also a very modern New York street scene and a French cafe scene I've seen before that I would love to own, but examples like these are few and far between. If you find a great kit, snatch it up because I haven't seen that New York one in years!
I wish paint by number would catch on again. In a way it did after an exhibit at the National Museum of American History in 2001-2002. Produced at the same time was a companion book called Paint By Number: The How-to Craze that Swept the Nation. Here are some other books and combination book and kits on the craft that are available.
Paint By Number Kit: Everything You Need to Re-Create 8 Vintage Masterpieces Complete Paint-By-Number Set
Painting By Numbers
Painting the Great Masters by Number
Painting by Number is definitely a great way to relax, which is why it was so popular in the post WWI years. They do not need a lot of skill or talent but still impart a feeling of accomplishment when done and hanging on your walls.
Here are some online sources where you can still buy quality kits:
Dimensions/PaintWorks - the best collection of the bunch Hobby Masters New England Arts & Crafts