here's a small few.
ps. Shapeways is by far my favorite, great forum, usability, price and quality. (though i would like a price reduction on each multiple order, especially silver
..... GO SHAPEWAYS!.
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https://www.turbosquid.com - has quite a few free models and priced ones.
c4dcafe.com - a great community for those using/learning c4d, pretty much instant feedback to questions.
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https://www.imaterialze.com - shapeways only real competition, prices slightly higher but offers gold and titanium. also they hallmark the silver rings.
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https://www.rapidprototypeservices.com.au - for those Australian's out there these guys do next day turnaround for metals, the first setup is a little bit more expensive than shapeways but if a few items are ordered it works out a lot cheaper. metals priced my the gram including 9, 14, 18, 22k yellow, rose and white gold, silver, palladium and platinum. they also mail out wax prints (though the wax is a bit pricey) ... also the quallity of the prints is just amazing!!, though sprues have to be removed by you and they offer no polishing. The negatives for RPS is no forum, no gallery, high setup and high model rejection.