Where I can buy/rent digital versions of Studio Ghibli films? To be honest, I’m mostly interested in Hayao Miyazaki’s films (My Neighbor Totoro, Princess Mononoke, Ponyo). They aren’t available on Netflix, iTunes, the PS3 or Xbox VOD services despite other Disney-owned movies (Pixar movies for example) being available there. Has Disney ever put any of Ghibli's films in any digital distribution service?
For whatever reason, Disney seems to prefer more traditional distribution methods. Going to Disney's Studio Ghibli page, I could not find any information on purchasing these in any digital format. All of the links they had were to purchase physical DVDs or Blu-rays. If they were selling it in a digital format officially, I imagine they would have included that on the website.

As for streaming services, you've listed most of the relevant ones, which probably means it isn't being streamed anywhere legally. I know for a fact that Ponyo was available at one point for streaming through Netflix, but what movies these services offer can vary on a day-to-day basis. I can't tell if any of them are available for streaming at the moment, but even if they are, it would likely change in the near future anyway, so your best bet is just to check with whatever services you do use and see if they have it. If not, your only legal option (at the moment) is to buy the DVD/Blu-ray.