said by SandShark:
I wish I could help you find one, but I have no idea where you'd buy a NID. Any reason why your in-laws won't get the phone company to install a NID?
The OP is not asking where to buy a NID but instead where to buy a DSL filter which fits inside a NID such as this one.
»www.suttleonline.com/catalog/dsl ··· ter.html
Sorry OP, no idea on finding one in the Houston area.