I really like to buy previous years’ calendars off of calendars.com or ebay. You can often find calendars for under $5. Right now we are studying Michelangelo, though, and I found a great hardcover book with all of his works from a used seller on Amazon for about $15.
I find them at my local used book stores, the Friends of the Library book sales, and I LOVE the SCM Picture Studies sold here in the bookstore. They are beautiful and fit right into my display frame. I also use old calendars.
I put one as my desktop background (the one we’re studying) and then my screensaver rotates through our past picture study pictures, or through a selection of works by that artist. Free and easy!