Where can I find a good fiberglass Eames chair?  

They are all over the place....
They are all over the place. No point in buying replica when you can get a real one for less.
Since its gonna be a office chair are you just looking for a side shell with a rolling desk base? Those go for about 80$ in the non rare colors

Thanks, but where?
@difeliciantonio thanks for the quick response. Every where I've looked I've only found them for $350 or more, even for the replica fiberglass chairs. I'm not talking about the plastic ones, which are super flimsy and cheap looking. Those I've seen all over for $80 or so...

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If you seek it, you will eventually find it. One day, after a billion fruitless trips to Goodwill... KABAM!
neurotic shopping leads to luck, in my experience-

second.
I'll second that. Estate sales tend to be good places as well--if you're able to get there early. Even yard sales for that matter. Like objectworship said, it's the vigilance by which you shop that creates the luck for you. Happy hunting.

(furrows brow)
Of course, "good one" and "replica" are mutually exclusive terms when discussing this subject...
and after reading your other (spam?) post re: the Eames lounge knockoff you purchased, I suppose you ought just sign into ebay and get it done
but I've found shells on the side of the road, and I've known others to, too!

I saw a few on eBay for...
I saw a few on eBay for 80-135 the other day . the armshell do bring more money than the side shells And age and color can change the prices