You probably won't have a chance of getting reservations in the valley itself unless you call a year in advance (unless you're willing to take a primitive "tent cabin" - I'm not sure how soon those get booked up). Yosemite View Lodge in El Portal is the closest place outside the park (and you'll have no problem getting reservations for September if you reserve a reasonable time in advance), but it's pretty expensive. There are less expensive places in El Portal (just a few minutes further), and also in Mariposa & Oakhurst (towns somewhat farther). Many of the available lodgings outside the park can be found at the following web site:http://www.yosemite-motels.com/
(Same link as Riverman's, but for all half dozen or so places instead of just the one.)
In September, the highway through the park's high country will almost certainly still be open from the summer season, and there are places to stay outside the park's east boundary at Lee Vining. However, while they are convenient for the high country, it's a loooong drive to the main valley from there. If you haven't already spent considerable time in Yosemite before, you really want to see the valley. However, if you get tired of the crowds there, and can't hike an hour or so up one of the less popular trails to get away from them, the high country is a nice retreat.
I hope you aren't planning to visit over the Labor Day holiday weekend, September 1 through 3! That's probably the busiest weekend of the year in Yosemite, and they have been known to close the park partway through that weekend due to overcrowding.
P.S. I'm staying at the Yosemite View Lodge this coming weekend.