In the Houston area, O'Ralley's (sp?) and Pep Boys carry them. They are made by Walker and are considered to be a good low cost muffler. You should be able to get a set of
Dynomax Turbo for around $30 each.
Go to www.dynomax.com and look for a list of dealers or contact them and ask where to go in your area.
I have DynoMax Super Turbos on my 5.0 and Race Magnums on my BOSS. Very happy with both, great sound and performance.
I've had Dynomax SuperTurbos for 14 years with no problems and get a lot of compliments on the sound. No droning, as Flowmasters do. I've also had Dynomax headers for seven years. Perfect fit, no problems.