I've looked for it too. Its not available. But there is some pro mix potting soil available it Christina's Flowers located at Preston/George Bush.
I used the web site for the Pro mix and it took me there.
I went to check it out last week. They do not have the seed starting mix.
I asked the girl working there and she said they don't use it. They start all their seeds and plant all their pots with the potting mix.

Here is a link that might be useful: Cristinas' Flowers