what is the best web site to buy tickets to mexico if i want to travel by bus and leaving from Houston tx?

Also, I will be arriving in Morelia Michoacan.

Grupo Senda (link below) is probably the way to go... website is in Spanish, but they do offer service from Houston to Morelia. My parents are originally from Morelia too (we live in Dallas) and they use to catch Greyhound to Nuevo Laredo and then catch a bus at the terminal...

There are 3 more links below of Buses that go to Morelia from Nuevo Laredo... (ETN, Omnibus de Mexico and Estrella Blanca), all offer Executive Class service. I personally recommend ETN, best overall... Hope these helps...