Gasket part numbers:
2x - A910E1475F - plenum cover to plenum backplate gasket
1x - A910E1474F - diffuser to plenum backplate gasket
I have asked Joe at JAE what the cost of the gasket is and will update.
Plenum Cover bolts:
The holes in the back plate that has the threads are thru holes (not blind) so bolt length is not super critical. The plate will be 25mm thick so you will need the following as Derek has listed above.
2x - 80mm Home Depot = Store SKU # 529111 $1.77 each.
8x - 110mm or 100mm (105mm is not a common size) Home Depot = Store SKU # 519251 $2.12 each.
Bolt Depot is a superior hardware vendor and has this in stainless:
I chose HD because there is usually 1 next door to any other store in the shopping area. Of course, Lowes has hardware as will most hardware stores. I personally will use Stainless Steel (Bolt Depot) hardware which can be found at these stores and online. I will buy 110mm length, with washers and if they stick thru the back plate and don't look good, I will grind them shorter. This is pure personal visual preference.
As for using a sealant on the gaskets, without knowing the flatness of the parts, a sealant is probably going to be needed. I love 3-Bond for gaskets, available locally as Permatex Ultra Grey sealant. It should be used SPARINGLY on any gasket and by that I mean a really thin bead of sealant. You are NOT caulking the tub to wall joint. You want a bead the is thin like a small string laid down on the center of the gasket and on the inside of the bolt holes. Once assembled, any squeeze out means too much sealant. The gasket is the 99.9% seal for these applications, the 3-Bond is just to take up any gaps and provide support.