When you said TransAm I thought that assumed the TransAmerica route, but since you asked about maps maybe you mean that in a more generic sense. Either way Adventure cycling is a good source for maps. They assume that you will follow their route and don't cover much area to either side of their route so if you want to improvise you need to pick up state maps along the way. On the other hand their maps negate the need for any route planning.

Their maps contain a wealth of info including the locations of all types of services along the way (libraries, stores, post offices, campgrounds, parks that will let you stay for free, churches that will let you stay, etc). They also have some general info about the area that you will be passing through. That info is generally very good. Sometimes there are changes, but these are addressed in the addenda that you can download before leaving.

As far as companions...
You can post here, on the crazy guy forums, and with adventure cycling (I think you need to be a member). On the other hand many just meet folks along the way and ride or camp with them as long as it works out. On the Trans America Trail you will meet other riders. When I did the TA I started a good bit earlier so I am not sure of the number of folks you will meet, but judging by the folks we met going the other way there are a lot of riders starting as late as you plan to.

If starting in July definitely go West to East the weather will be much more likely to be pleasant that way.