Fire bricks come in different sizes so a generic ‘150’ may or may not be enough. Insulating sheet, i got mine on eBay but other routes are available. Insulation blanket is a little more specialist but readily available. For a one-stop-shop try Kilnlinings where the owner is very aware of the needs of WFO builders.
The ‘wet’ ingredients can be had from most building supply places, Wickes, Jewsons etc but you really only need Sand, Cement, Lime, Clay to make ‘homebrew mortar’ which is all you’ll need for a WFO and is superior to so called fire-cement in many ways.
To reduce costs even further, consider using ordinary building bricks for your dome, leaving firebricks for the cooking floor only. Try reclamation yards for old ‘solids’ i.e. no frogs, holes or indents and remember that as you’ll be cutting them in half you can buy damaged ones even cheaper. FWIW i got my bricks from a neighbours skip for nothing.
On the amount of bricks, I've just finished my oven, 900 internal. i used 76mm fire bricks and I tapered them on 3 sides for the dome. In total I orded 270 bricks and I've got 30 left. I first only orded 210,so had to stump up extra on postage for the extra bricks. Wish I had of know more about the homebrew before i started, would have saved some extra cash, but as it is, I used 65 kilos of vitcas outdoor oven cement, These figure can change though, if you don't taper the bricks you will use less but use alot more cement.