hot-hen: It depends on how many chicks your looking to get. Most hatcheries (that deliver to you) require a minimum order of 25 chicks. MyPetChicken lets you order 3 chicks or more but the shipping is pretty steep.
Your best bet is to find a local feed store that has sex-link chicks available. You could check with Jones Feed or V&V. They may not be offering chicks now but would in Sept.
I do see hatcheries/private sellers on craigslist that will occasionally offer sex-link chicks. Some are excess chicks from those 25 chick orders.
You could also try to go in with someone on an order of chicks.
Lots of options, but unless you travel to somewhere cooler to get them, you best wait until it's cooler to ship chicks.