From Islamabad or Rawalpindi one can reach Texila within half an hour by local wagons, buses or texi. You can hire a private car to visit Texila or contact Punjab tourism corporation offices in any major city of Pakistan and they can arrange guided tours to texila. I just asked the concierges at my hotel the Islamabad Serena in Islamabad for transport.
Basically it is a half day trip from Islamabad/Rawalpindi, and to get there you may need your own transport. Buses also go there but then you will be stuck in one location as majority of the sites are far apart. Therefore better to have your own car/transport to enjoy various sites at Taxila