Portable Air/Fuel gauge?

NO, WB sensors have to be hot to work properly. Tailpipe sniffers are a much different deal. You CAN find them on ebay but generally, more expensive. Many of the older ones are not really "wide band."

Saving money on an O2 bung:

Go to your local "real" nut and bolt store and buy some 18mm nuts, don't remember the thread, but same as Ford 18mm spark plugs. I think I paid less than a buck and a half for nuts

And if you have an old 18mm plug, you can use that for a plug if you want to remove the O2