They are cheaper than I expected (I only checked the top result and it doesn't tell you delivery, presumably until the last minute).
Strongman Atlas Stones for Sale - Strongman; Strongwoman; Functional Training - Sizes from 8"-22" 9-280kg. Used throughout the UK. Discounts available
kingheavy.co.uk
Novice Set - 30kg/66lbs, 50kg/110lbs, 70kg/154lbs, 90kg/198lbs - £239.95
Intermediate Set - 70kg/154lbs, 90kg/198lbs, 100kg/220lbs, 125kg/276lbs - £349.95
Advanced Set - 100kg/220lbs, 125kg/276lbs, 150kg/331lbs, 175kg/386lbs - £399.95
[T]he [World's Strongest Man]
Atlas Stones... increase in weight from 100kg/220lbs to 160kg/353lbs.
Tom Stoltman holds the record for the heaviest Atlas Stone lifted over [a] 48 inch bar at 286kg (630lbs). Tom also owns a 304kg (670lbs) Atlas Stone which he was planning to break his own record with. These two stones are 22 inches in diameter.