Ive generally got them from car boot's and 2nd hand shops, there was a 2nd hand book shop just off soton high street a few years ago that used to have quite a few.
I pay anything from about 50p to £20 / £30. Funnily enough, my prized atlas, which was printed in 1899, cost 50p from Bursledon car boot !!! Its worth considerably more !!