have you ever tried saying to your local fireworks supplier "I've seen this at XYZ Supermarket, what can you do for me?"
Not everyone lives near an all year round fireworks shop. There are none at all in the Manchester, Liverpool and Central Lancashire area. Also if you live in the North East there isn't a single all year round fireworks shop. If you live in somewhere like Hexham, Alnwick or Morpeth your only option around bonfire night is the supermarket or the newsagent.
As for me, I am happy to travel the country to visit fireworks shops- I love it. But for many people that simply isn't a viable option.