In Texas, the Comptroller of Public Accounts administers this and you can download a blank form at
window.state.tx.us/taxinfo/taxforms/01-forms.html
Look for(you want the second one)
01-339 (Front) Texas Resale Certificate
01-339 (Back) Texas Sales and Use Tax Exemption Certification
The Comptroller's staff will tell you again and again that there is no such thing as a "tax exemption number" and to give businesses a completed copy of the form. Nine times out of ten, the business will insist on you giving them your "tax exempt number". You do have a taxpayer ID number issued by Texas (a version of your federal EIN with a couple extra digits). Give them that number - everyone will be happy.
Personally, I just carry PTO business cards (printed ourselves) complete with our ID number. Actually, I frequently have NOTHING official with me, except I do keep the ID number in my address book. The businesses who really want the 339 forms usually keep blank copies at the customer service desk. A lot of them have their own internal form.
Once you get the hang of it, it's a pretty easy process.