Hi,
Check with some TR3 racers/drivers like Ken Gillanders at British Frame & Engine (
www.britishframeandengine.com) They'll be the most likely to know of any sources. On the other hand, a lot of racers have made their own molds and panels over the years.
You specified a U.S. source, but...
I asked Honeybourne Mouldings in England a while back and they didn't have a U.S. distributor. Perhaps that's changed. They seemed a good source of some special items like a replica TR4-250 hardtop, GT6/Spitfire "LeMans" hood and TR3 "Beta" fenders. Prices were reasonable. Shipping wasn't too horrible, even from overseas.
https://www.honeybournemouldings.co.uk
Other sources in England include
www.racetorations.co.uk, and
www.revingtontr.com. Revington seemed to have the most extensive selection, at least for TR4 which is what I was looking at. Last time I looked,
www.cambridgemotorsport.com didn't list any f'glass panels on their website, but probably offers or can get some.
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Whoops, I just corrected the Honeybourne URL, try again.