I have found some vintage ones on ebay. They were pretty reasonable and had good pictures that could be used over and over.
Virginia

I've used online and any cheapo discount store, junk store, etc. where the toy cars aren't hotwheels and the underwear shrink. They work nicely on grid paper for cross stitch patterns as well.

my mom used to hold new coloring books until she copied all the patterns she liked out of them..... then we could have them to color in

she had everything she traced in 3" binders, by category.. i should be so organized!

There are also a few educational and teacher sites on the internet. They have lots there for different ages.

Look online. Just google "coloring books" you can even put in a subject search like birds or flowers. great fun.

dollar stores that have the "cheaper" books usually have the simple drawings in them

for $1, bigger 1s are $2-$3 but they are usually good for quite a few applique pieces :D lol
I go to the dollar store, buy a bunch of the easy ones for my youngest 2 DDs.... then i borrow each of them for a couple hours. lol

Look at Goodwill and Salvation Army stores and church thrift outlets. They just don't make the simple color books now-a-days.

some times at the dollar store. God bless. Penny

I purchased a design source book for crafters at Barn's and Nobels and it is fantastic with hundreds of images and the book is over an inch thick. It only cost me $9.99. It is a book that is really, really good to use for applique. Might be able to find it on amazon.com or just search for design source books, there are many out there. I also have iron on transfer books that I use for designing.