I'm currently using two pix 506e firewalls; one here, and one at my colocation. I have a third as spare, but no spare power supply, and it's the power supply that's most likely to go toes-up.
I have bought a spare power supply, but it's coming from the USA, and might take a while to get here. I've also bought another 506e with a power supply, so when either of those get to me, I'll feel that I have a fall-back.
This is only a short term solution. In the longer term, when my ethernet link finally arrives (it's taken eight months so far and counting), I plan to switch to a Pix 525, which has a great user interface, and when I'm happy that's working well, I'll switch to a pix 515, which has a DMZ, and will be my only firewall, with the Pix 525 as my backup in case of failure.
Today, I had a look at the failed 506e power supply, in case I could repair it. Maybe the problem is just a blown fuse? Sadly, no. But as I looked at the 506e power supply, I could see that what it gave was ground,+5v, +12v and -12v. And that's exacly what you can get out of an ordinary computer power supply.
So I checked around the web, and I found this and this, which show how you can adapt an ordinary PC power supply (costing under £10).
I have an even better idea.
If I cut off the cable from the dead pix power supply, then I have at one end, the plug that goes into the pix, and at the other end, eight wires. And there are also these, which I have several off in my spares box.
But since I have two 506e power supplies on their way to me, there's not really any point in doing it. Pity; but it would be a neat fix, if things go south before the power supplies arrive.
By the way, here's what you do if you've just bought a second hand pix 506e (or any other pix), and you don't know the password. You connect to the serial port at 9600,N, 8, 1, power the pix on and when it does a 10 second countdown during startup, you hit "esc". That gets you the "monitor" prompt. Get the PIX password Lockout Utility from here. And that page also tells you how to run it.. After you've done this, the password to the unit is "cisco", so, of course, you should now change it, and create an enable password.