Don't cut corners just to save a few pounds. Always buy fireworks from a reputable shop to make sure that they conform to British Standards. This means that they should have the British Standard written on the box.
Sometimes shops open up for a short time before Bonfire Night but these may not be the best places to buy fireworks from. Staff in these shops might not be very knowledgeable about using fireworks safely and their fireworks might not meet British Standards.
Don't buy fireworks from anywhere you're not sure about, such as the back of a van or from a temporary, unlicensed market stall.