Aside from Milligan's on Plumbe Street, does anyone know where I can buy firebricks?

Having a go at building a pizza oven. Lots of places to buy online, but expensive delivery charges. I have a price from Milligan's but they're currently closed for a holiday.

Try giving greengates in Accrington a call?

Clarkes in Nelson the works greatest DIY shop
Proper old school they even repair shoes at £17 sole and heal.

Hi there, do you actually need fire bricks?
I work at Accrington bricks and know that the firing temperature is around 1000°c.
A quick look on Google suggests a pizza oven optimum temperature is only 260°c so a normal brick will easily take that.
Also, I have built a barbecue using paver bricks and the heat has had no effect whatsoever.

Could try Valley supplies in Rossendale.

If you use fire bricks your pizzas will likely as not end up covered in firebrick dust. They are prone to becoming very friable after a while.