Ed
The diodes are 3AF4 (which means 3 amp 400 peak inverse voltage rating).
Radio shack is pretty much out of handling components and I don't believe they are supplying these anymore.
You can get them at
www.fairradio.com new and surplus 4.--/2.50. They are available at most jobbers and from Phillips semiconductor. Just tell them you want 3AF4 or equivilant. Larger current up to 5 amps or higher PIV won't hurt as long as they are BOTH THE SAME.
Let me know if you still can't get your hands on some and I will help out.
Don't forget to change the CAPS at the same time. They should be 450 - 500 MF at 100 volts (electrolytics). Careful how you put them in because these are polarized and the end with the band or indent is the negative side. Observe the old ones as you remove them. Same for the diodes.
Bob