Russell,
You really must replace those fittings. If I was making them out of 2024-T3, I would make them 50% thicker than the old fittings. Even if I made them from 7075-T6, I would make them a little thicker. What you are seeing is due to the fact that the higher strength aluminum alloys are much more susceptible to corrosion than the softer, weaker alloys.
There are lots of places where you can get 2024 and 7075 plate. Because you need just a little bit, you might want to try just dropping by any local machine shops or smaller custom fabrication shops in your area and see if they can sell you some of their excess (scrap). Look in your local Yellow Pages. Sometimes they will just give it to you.
I get much of my aircraft related stuff from Charley Vogelsong in Pennsylvania. He carries most anything you will ever need. He is terrific, and very fast. Give him a call:
The Dillsburg Aeroplane Works
114 Sawmill Road, Dillsburg, Pa 17019
Tel: 717/384-5865 Fax: 717/432-4589
Also you could try
http://www.airpartsinc.com/ . They bring lots of small cut-offs to Oshkosh and Sun n' Fun. Give them a call.
Also there are the big homebuilt Aircraft suppliers like Aircraft Spruce and Wicks. I haven't ever tried the Metal Supermarkets, but they have stores located in the South and should have 2024-T3 in small quanities. I even found good aluminum scrap stock at local junk yards. Some will separate the better, marked stuff and sell it by the pound. That's how I got the 7075 I needed years ago, and paid next to nothing for it.
The stuffs out there, you just have to dig for it. And REMEMBER, if you are shopping at non-aviation suppliers, DO NOT tell them it's for an airplane or Ultralight! Many will not have anything to do with you if they suspect it's for a plane.
Chappy