I went through this a while back trying to find new drain fittings for my galley sink. I ended up going to WM as they happened to have the parts in stock locally... but prior to that, I was going to the Defender site, and then to Forespar to look at Marlon and Groco to look at bronze.

The local lumbers shop had similar sizes, but not the materials I felt safe using... PVC and brass and thin wall junk metal. (plumbers don't have to consider that the house they are working on could sink if the parts fail.)

Look at this site to see if they have the parts you really want... http://www.groco.net/fittings.htm

How to get them is another story... I think Defender carries some.

The tailpieces were the big issue... finding the right threads and barbs for the sizes I wanted. Ultimately I modified the sink to fit a more modern drain design... one that used stainless grates vice rusting steel. I used bronze tail pieces below the water line, and marine PVC Ts to tie the drains together above the waterline... avoiding any household type items.