The International Group of PME (the organization formerly known as Psychology of Mathematics Education)

Here is a link to the IGPME website.

You can create a log-in for yourself, and that will give you access to the full PDFs of the PME proceedings from 2013 and 2014, and 'print on demand' paid versions of the proceedings from 2015 and 2016.

I have hard copies of the proceedings for the PME conferences where I've presented -- and Cynthia Nicol has more. The UBC Library may have hard copies and/or electronic access to PME proceedings as well.

Through Google Scholar, you can access articles from the proceedings of PME conferences going way back.