Well, there's a big difference between wartime production F-S knives and modern production or custom knives. May be beneficial to think about why you want it. If you just like the design and want one to fidget with on your desk, then maybe a nice modern production one will suit you. If you're thinking maybe to start collecting, then rather than getting a knife, I'd recommend picking up a copy of a decent collector's guide and learning a lot first.
Most of the original knives are sold by collectors or auction houses. There is one fellow, Roy Shadbolt, who maintains a good informational web site and often has some for sale there. You are looking at $800-$4,000 depending on the pattern, condition, and markings.
For reproductions, Atlanta Cutlery makes them in the <$150 range. Sheffield Knives and J. Nowill & Sons in the UK still makes current production Fairbairn-Sykes knives, but it is hard for me to say what the pricing is given the currency differences and that they don't seem to be in stock or carried in a lot of places.
It also depends on a bit on whether you want a reproduction that's constructed in an identical or broadly similar manner as the original with brass handles. Fox makes one with turned wood handles with the Third Pattern grip grooves instead of the First- or Second-Pattern knurling. Les George has a collaboration F-S inspired dagger with Spartan, but it is a full-tang knife with composite handle scales. That one is likely a better
dagger than the originals, but it only loosely resembles them in appearance.
This one is a custom by Matt Martin of Vehement Knives based on the Second Pattern F-S. I believe there are nine of these, two were at BLADE Show, and he had I think six or so at DLT a few years back for the first Twelve Days of Christmas special event sale. The 1918 Trench Knife is by Les George and Andrew Bawidamann. The M4 bayonet is a Camillus WW2 production knife.
