Second-hand shops like Oxfam or similar places usually have pretty cheap DVD sections, and it's also worth checking out local flea markets or antique fairs where they sell bulk lots. Online, sites like Wallapop, Vinted, or even Facebook groups dedicated to movie collectors have stuff at better prices than stores, though you need to be careful about the product's condition and shipping costs. Some old video rental shops that are still around are clearing stock at half price, so if there's one near you I'd recommend checking it out, and also look at the sale sections in discount stores like Action or outlet shops when they have sales.