Remember that the places where you are looking to live (Oakland, Shadyside, Squirrel Hill) have a very specific rent cycle.
Right now, the best rent deals are for summer sublets, as we have many students who are still looking to sublet their apartments over the upcoming summer months since they will not be taking classes this summer.
From my experience years ago as a student in Pittsburgh looking for an apartment for the following academic year, the absolute best time to get an apartment for the following school year is February, March and April, as landlords want to lock down students for next year before a large portion of the students leave for summer break. Obviously - you were not able to take advantage of this.
But - from now through August, you will start to see more and more individual apartment deals popping up, as people's plans for the next school year change (i.e. of the large number of people who have signed leases for next year, people will drop out of school, people will take jobs and not go to school, etc.). In my case, I ended up finding a very affordable but solid quality apartment as a Pitt grad student in the August before I started school (we started around August 28th).
So - don't stress too much about not being able to find a place right now - especially since you are not coming until August. Just be diligent and check Craig's List every day (as well as CMU's housing office listings - assuming that they have listings on a website).
Here is Pitt's off campus housing office (which has an apartment listing section). This is worth checking frequently as well:
http://www.ocl.pitt.edu/
Also - I'm not sure why you are focusing so heavily on Squirrel Hill. Don't get me wrong - it is a nice place to live, but Shadyside is a great place to live and is very close to Squirrel Hill.