I wish I knew where to buy a larger dress form! I’m in Australia, so my avenues might be different to yours but I might try looking in ebay for local listings, or even local garage sales. New ones can probably be bought from sewing shops where you can buy fabric and notions.
With regard to plus size sewing patterns - I’m afraid there is not much at all. The BurdaStyle site reckons they have plus size patterns but there’s not that many and the site is a pain to navigate. From memory Simplicity had a plus size pattern range that was ok, not fabulous, but ok. Most patterns relied on using stretch fabric which isn’t that easy to work with unless you’ve got a machine with the right feet and settings.
I’m sorry I’m not much help - I would say that I probably have medium sewing skills because I have found it so difficult to find patterns and sewing resources. I have bought “Fit for Real People” and I’m hoping it can help me with pattern drafting for my size!