Booking rooms Online
You can check availability of rooms at the guesthouse online and also book at http://www.ttdsevaonline.com/
Rooms are booked well in advance. It is pretty difficult to secure accommodation at Tirumala Hills.
Article in Times explains why it difficult to get accommodation at Tirumala.
Tirumala temple Call Centre
You can find availability of rooms from TTD run call center
Contact Number :0877-2277777, 2233333 Toll free No: 18004254141
There are no private hotel accommodation at Tirumala. Accommodation options at Tirmala is restricted to guesthouses managed by TTD(Tirumala Tirupati Devasthanams)and other religious institutions.
Guest houses of other religious institutions
You can book rooms at Karnataka Guest House as explained in that post.
http://www.travelpaisa.com/spiritual/275-tirumala-hotels.html lists phone numbers of various mutts that offer accommodation facility
Stay at Tirupati
Your best bet will be stay at Tirupaty where there are plenty of hotels. Reaching Tirumala from Tirupati is easy - there are frequent buses. you can probably look to hire a taxi.