Single handle shower tub knob

Hi, My single handle shower tub knob broke into pieces. Its the clear plastic knob from Gerber . I couldnt find it in the local hardware stores. Also I couldnt find a suitable universal knob, look like I have to replace the whole set. But I dont know again what should I look for to find a suitable replacement. The Gerber doesnt have much choice or a good website to navigate and find a replacement part. Appreciate if anyone point me to the right direction or a part supply on the web where we can find replacement parts.
TIA
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knobs, and includes Gerber on the brand list. Other possibility is a 'real' plumbing supply, one that caters to the trade. May have to take off work to go there- they are usually closed by 4:30 or so in most towns I have lived in, since plumbers pick up their parts first thing in the morning. If you can get the old knob remnants off to take in with you, that will be a big help.
aem sends....
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