Sound the bugle, hopefully the Third Foot And Mouth will be along shortly to assist here.
The details of Third Foot & Mouth kit, including suppliers, can be found in the Carry on Up The Khyber
thread. Khaki tunics from What Price Glory (good price, great quality and quick shipping)
Surplus red guards wool tunics turn up quite frequently on ebay, though the larger sizes (40" chest and larger) tend to be quite scarce and so command higher prices. A colleague at work has a 38" (ish) chest Scots Guards tunic going spare, sadly it was too small for my 42" chest :'(, but if anyone's interested, please PM me The wool tunics are rather warm to wear, especially in summer or a crowded con!
Surplus trousers are again common on ebay and elsewhere, I've a pair of RM dress blues (thin red stripe) and RHA overall trousers (wider red stripe)
White Pith sourced via Tinkers Emporium (he also does them in khaki).
Good luck with your search!