Global Tooling certainly has a very good reputation for third-party knives at less than OEM prices, as well as carbide knives for many models. If you go this route, be sure to carefully measure your existing knives for length, thickness and *original* height as there are many common combinations. Many jointers, but not all, for example, have knives that are 1/8" longer than the jointer's claimed width to ensure the full cutting width is available to the user when rabbeting, while others are exactly the claimed width; some are 1" high while others are 3/4" or 3/32" thick versus the more typical 1/8", etc. Like many things in woodworking, accurately measuring first avoids the hassle of assumptions.
However, if you are looking for OEM knives you can order/buy from Woodcraft if it is a model they carry, or if it is a common knife dimension. Also, Amazon carries OEM and generic third-party knives for many manufacturers at better than typical retail prices.