I don't know about Walker, Lisa is right about Care. Colonial and Care both list respiratory equipment and oxygen on their web sites.
Care Medical: http://www.caremedicalequipment.com/
Your MIL should work with her local oxygen provider to get it all set up for Orlando. She will need to have prescriptions listing the type of equipment that is used to deliver the oxygen and the flow rate.
If you are staying at an on-site hotel, you can get help arranging this from WDW RESORTS/SPECIAL RESERVATIONS
(407) 939-7807 [voice]
(407) 939-7670 [TTY]
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