Like many here Amazon and Kindle rank high on my book vendor list.
One thing I do fairly often on Amazon, is check out if the book I am interested in is available as a 2nd hand boook via one of the many 2nd hand used book vendors on Amazon. These are frequently smaller private bookstores, some are thrift shops and some are selling used library books. If I am just looking for an entertaining fictional story, or maybe a Photoshop book, or another book I may not want to keep forever, then a used book in good condition can be pretty inexpensive. Some will ship for free with PRIME, but most will add the mailing charges for delivery which is usually about $4 bucks. Shipping seems to have gone up a buck recently.
On Kindle many of the books from the 19th century that are beyond copyrite protection are frequently free, or just a buck or two. I purchased "A Journey in the Back Country" by Frederick Law Olmstead for 99 cents. This book described Olmstead's journey through the southern US in the late 1850's before the Civil War. This was the third volume he wrote about his extensive travels in Mississippi, Alabama, Tennessee, and the back country of the Carolinas and Virginia.
He wrote a separate volume about his travels in Texas. Just imagine travelling alone, on horseback or foot, as a Northern reporter for a New York newspaper, through the slave states and sleeping on used straw beds in taverns at night......
"Bleak House" by Charles Dickens was free.
I found "The Ancient Monuments of the Mississippi Valley" by Davis and Squier published in the 1840s for $1.06 It describes an extensive survey by the Smithsonian Museum to study the mound builders forts in the Ohio Valley 1830s shortly after the creation of the NorthWest Territories.
Many of these older books are not even available at my local libraries...