There are several places online that sell the PreSonus add-ons. For example, B&H has the CTC-1, Lunchboxaudio has all the Fat Channel add-ons, and so on. There are other outlets as well, like pluginsmasters.com, proaudiostar.com, and a couple others I forget at the moment. Search on what you want to buy, and then wade through all the irrelevant hits (like links to the PreSonus shop). You'll almost certainly be able to find what you want, and it will still work in Studio One.
I agree PreSonus should carry the add-ons. However, the reason why the PreSonus shop no longer sells my Studio One books is because they turned off the store for all 3rd-party products. So maybe licensing or third-party issues became relevant when PreSonus was purchased. (Something similar happened with Cakewalk changed ownership, and they couldn't bundle certain Sonar plugins in Cakewalk by Bandlab.)
Hope this helps.