can you recomend a travel agent that can organize a tour of the atractions in the Jiayuguan?

To be honest, it is suggested to travel on your own and the transportation is convenient to reach there from Lanzhou.
If you want to join a group tour, you can ask TravelChinaGuide for help. You can send them emails at travelchinaguide.com|tours and they will arrage a nice route before you arrive in China.