Mongolia is a difficult place for independent travellers in wheelchairs. While the infrastructure is getting better, pavements are rough, and buildings and buses are generally not wheelchair-accessible. It's best to travel with a private guide who can assist with transport. Travel to Ulaanbaatar and 4WD trips to places such as Khustain National Park shouldn’t cause too many insurmountable problems.
If any specialised travel agency might be interested in arranging trips to Mongolia, the best bet is the US company Accessible Journeys (www.disabilitytravel.com) in Pennsylvania. At the very least, hire your own transport and guide through one of the Ulaanbaatar agencies. If you explain your disability, these agencies may be able to assist you.
Download Lonely Planet's free Accessible Travel guides from http://lptravel.to/AccessibleTravel.