Maas Nursery (Seabrook, TX) - They don't have a huge selection, but this is one of the better/cooler nurseries I've been to. There is a two (plus) story tall Celeste tree on the premises which I a source of some of there trees. The rest is brought in from wholesalers.
RCW Nursery (NW Houston) - For those who remember hlyell, he asked me to pick up several trees he bought from here and ship them to him. They had a nice selection of decent size trees sourced from Brazos Citrus Nursery.
Urban Harvest (01.19.2013) - A once per year sale benefitting a nonprofit, community garden / educational organization. The last time I went, they had a line which snaked around the entire sale then ended at the check out. When the line got to what you wanted, you grabbed it (if there still were some) and drag it through the rest of the sale. The sale this year is at a different location than when I went so things may be different. Get there early for best selection. Some trees are in containers while others are bare rooted, field grown trees.
Harris County Master Gardners (01.26.2013) - It is like the Urban Harvest sale (it would not surprise me if the same nursery supplied both), except for it is more of a free for all. Also, there are lectures on various gardening related topics. Yaddayadda once mentioned a Ft Bend County MG sale, but I have no info on this.
LINKS:
http://maasnursery.com/
http://rcwnurseries.com/rcw/
http://www.brazoscitrus.com/
http://urbanharvest.org/events/fruittreesales.html
https://hcmga.tamu.edu/Public/pubSales.aspx