Here's a list of those currently found, (my research for this only goes a page or two back,) please add more!

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Express Men
found by DarenHenry W

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Gap "safari shirt"
found online by singsingjohnny
http://www.gap.com/browse/product.do?ci ... =719536012
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Indy style shirt
found on Taylor Lautner by azuma
http://www.imdb.com/media/rm1754371840/nm1210124
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Express MK2 Fitted Cotton Shirt
suggested by riothamus and Magnum
http://www.express.com/catalog/product. ... =*&Mrsavf=*
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Indy style shirt
seen on Johnny Depp by Arch Stanton

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Sears Structure Convertible Cuff Linen Shirt
suggested by aichdawg
http://www.sears.com:80/shc/s/p_10153_1 ... 000P?mv=rr
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Eagle Military Shirt
suggested by Mississippi Jones
http://www.ae.com/web/browse/product.js ... rod5230252
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Tag Safari Safari Trail Shirt (along with other shirts at Tag

suggested by Snakewhip_Sable
http://www.tagsafari.com/item/mens-shir ... ong-Sleeve
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Of course, everyone also remembers the excellent Cabelas and LL Bean shirts.
For new guys, here is a good, but outdated, resource:
http://indygear.com/OLD_SITE/gear/shirt.shtml