I am looking at mags and +2 adaptors for M&P handguns. When I click on S&W and then 9mm, it only shows mags for S&W 5900 Series/915/910/659.

These are not the same mags for the current line of M&P handgun line.

http://shop.mec-gar.com/Magazines/Smith-and-Wesson/(model)/472/(caliber)/9+mm/(capacity)

No.

I'm like 99.99% they did at one point at least though.

Nope. Completely different, same as the Sigma mags were different from the 3rd gen and the later M&Ps.

A few months back I bought a 17rd M&P mag for my 9c because I bought on of those grip extensions for it.

I had not heard of Mec-Gar before my son told me about them and I could have sworn that was who I got this mag from. I might still have the recept, I doubtful I have it but I'm pretty sure that is where I ordered it.

I was looking at buying a couple more 17rd mags and I couldn't find them on Mec-Gar's website.

Mec-Gar is the OEM for factory M&P mags.  I don't believe they offer them for sale under the Mec-Gar brand.