First up, coloring books. I still recommend the Joanna Basford and Eleri Fowler coloring books, as they are high quality with beautiful designs. But if you're looking for something a bit more economical, I've found a few options. This coloring book was so good, that I even added the recommendation as an update to my original post over the summer. Parragon publishes a line of coloring books that are each 288 pages long and are priced around $8-12. The paper isn't quite as thick as some of the high-end books, but it's jam packed full of great designs. They range from geometric to floral to doodle-y--whatever your mood may be. The three images below are all from Coloring for Creativity. They also have a Coloring for Tranquility and Coloring for Inspiration.
In most of my coloring books there are mandalas or some take on mandalas, so when I saw a rack of coloring books on sale at our local Barnes and Nobel, I picked up Magical Mandalas for $5. The mandalas are beautiful and only single sided on nice quality paper so they could be framed or gifted very easily.
In addition to the great coloring books, I've also picked up some additional supplies. I still highly recommend the Prismacolor colored pencils (Verithins--hard for fine designs, Scholar--for medium multipurpose, and Premier--for soft saturated color and blending).
But I also found that their blending pencils come in really handy. They will smooth colors together and allow you to avoid pencil lines. They are well worth the price to pick up a couple.
If you're looking for a budget friendly colored pencil, I now have a second brand to recommend after Crayola. These Sargent Art colored pencils are similar in quality, hardness, and price to the Crayola pencils. So if you're looking for more colors to add to your collection, or if these go on sale...they're a pretty safe bet.
If you have any favorite supplies or books, be sure to mention them in the comments!