Who says MC supports must be listed?
You can use anything that the inspector will accept.
The majority of the jobs we work are MC and we do use some tie wraps.
This can be an issue above a ceiling that is used as 'other environmental air space.
Some inspectors say 300.22(C)(2) requires the tie wraps listed for use in environmental air space. (plenum rated) I personally do not see it that way.
For these inspectors we use bailing wire.
300.22(C)(2) Equipment. Electrical equipment with a metal enclosure, or with a nonmetallic enclosure listed for the use and having adequate fire-resistant and low-smoke-producing characteristics, and associated wiring material suitable for the ambient temperature shall be permitted to be installed in such other space unless prohibited elsewhere in this Code.
As a tie wrap is not an 'enclosure' IMO it does apply except for the ambient temperature part.
You can also find a bunch of different types of supports from Caddy or B-Line.