Here is a list of the vendors where most, if not all, of the components for the Progressive Receiver can be purchased. I have found each of the vendors listed to be trustworthy and reliable. Obviously, there are other vendors who stock the same components, and I have no financial interest in any of these companies.
Dan's Small Parts lists the 40673 Dual Gate MOSFETs, Mini Circuit Lab's SBL-1s, 2n3866, toroid cores and ferrite beads, small variable capacitors, and JFETs. Additionally, I purchased teflon coax and magnet wire from Dan's.
Ocean State Electronics also lists many of these components. They also list a dual gang variable capacitor that could be used in the preselector circuit.
I bought several of the crystals used in the converters at Digikey.
Ten Tec manufactures several enclosures that would be suitable for the Progressive.