Here are some links that have proven useful.
You may find the rest at: http://www.theinformedinvestor.com/TexasTa%20SaleandPropertyResearchToolsII.htm
1) Perdue, Brandon, Fielder, Collins & Mott, L.L.P. - little known site, but an excellent source for tax sale listings
http://www.pbfcm.com/tax_sales.htm
2) Linebarger, Goggan, Blair & Sampson, LLP (Linebarger Goggan) - excellent source for tax sale listings