can't find tub/shower size?

I have a home that was built in 1977. My plan is to replace the wallpaper in my daughter's bathroom with paint. After taking the wallpaper off I noticed that there was no green board lining the shower/tub. Thinking I could replace the tub/shower and sheetrock I went to Home Dept and Lowe's, but both did not have the size tub/shower that I need (58x72x30). My next thought was to paint a couple of coats of Kills over the existing sheetrock to protect it, but I've already noticed a few places where the sheetrock is crumbling. Any suggestions? Carlton.
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This is a great resource on tubs:
www.tubgirl.com
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You sick idiot!

existing
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