Try Bolt Bus. They're very good, with very low prices the earlier you book. You can book online. Amtrak also buses to LV, but the trip is longer due to more stops. Amtrak does give a 10% discount for people on disability though--only the person on disability, not fellow travelers. Since Bolt Bus if a subsidiary of Amtrak, it might also give the same discount.
Call or visit the Mega Bus website, you will find out there
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