Hi Kevin,
Welcome! After seeing so much about the trouble to locate firebrick in the UK and hearing how expensive it can be, I was intrigued enough to do a bit of 'net research and here are some starting points, at least (unless someone from the UK has better suggestions):
Handsworth Refractories - new refractory fire bricks, refractory recycling, redundant stock, refractory services, refractory site clearance & wrecking services DUDLEY DESIGNS
Handsworth, in Sheffield, as well as selling new firebrick, also has a quantity of 'redundant firebrick for immediate sale', whatever that means - possibly a deal! They may just sell wholesale but could likely provide you with the name of a supplier nearest you.
Dudley (with a bunch of interesting products) sells a wide range of sizes and their 2.5 ' thick bricks are sold in lots of 10 or of 50. The larger quantity breaks down to 1.58 pounds per brick - expensive by North American standards but perhaps not so for the UK. Seems you can have them delivered or pick them up somewhere in the Midlands - not sure what kind of drive that would be for you - there must be somewhere in Scotland that sells them!
A bit of an aside - nothing seemed to turn up specifically for Scotland but what was interesting is that manufacture of firebrick seems to have been HUGE in Scotland, up to the middle of the 20th century. There were some sites showing some old ones with stamped-in names that I'd love to get my hands on! Sadly, the industry seems to have died right out.
BTW, James, if you're reading this, "Firebrick Sources" might be a worthwhile category addition to the forum since it's often one of the first questions posted by people just getting started. Separate threads within the category could address specific countries, states or regions.