It is not mandatory that you use these exact transistors for the circuit. If you can find P and N types with hfe between 100 and 200 you should be in the ballpark. In some of Mr. Pass' older designs, you can see where he used MPSA18's for the same purpose.
By the way, Digikey has both the 550 and the 450 by the thousands. You can get them as singles for around $0.50 each. You can still get the MPSA18's from Digikey, and they are even cheaper than the Zetex components.