You guys bring up some really interesting observations. I have been around mustangs enough to definitely see the changes in parts supply. I have had 7 stangs...all different years and watched the availabilities. 66, 67, 73, 92, 94, 93, back to a 92.
I am currently eying a Explorer GT40 manifold.....just don't have background going in that direction. I hear/read its decent for a mild build.
I took a 14 year break from fox bodies, buying another one last year. I was expecting to see availability of used parts like when I left, not so. You can still find used stuff, just fewer and far between. The 87-93 fox bodies only represents 6 years of production and the population produced took a heavy beating. We now have classic cars that are getting rarer by the day. Look at CJPony Parts for your new stuff. Summit still sells all kinds of SBF stuff, they just don't list a fox body section in their magazine anymore, not enough sales volume, I'm sure.
I think the quality of parts has gone downhill too. Not necessarily things like trick flow heads, but replacement parts like a speedo cable. All cheap chinese crap. Even Motorcraft parts today seem to be cheaply made vs 10 years ago.
you can still get high quality aftermarket parts like a K-member, but if you need an ACT sensor it was likely made in some 3rd world country by a llama.