I've never used on personally. I've noticed that they can differ from site to site.
I'd suggest contacting a Surname Organization for your surname or nationality and get the correct one from there. Search on your surname and see what comes up.
You may want to read this first:
http://www.ancestralfindings.com/real-truth-behind-coats-arms-family-crests/
You have to realize that for England - arms were awarded to individuals not names. So my Samuel Peploe - Bishop of Chester was awarded a coat of arms but only his direct descendents are allowed to use it - not cousins or other that have the name. The crest is a decorative add-on to the coat of arms.
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Here's a bunch that I found from doing a basic google search...
http://www.freecoatsofarms.com/catalog.html
http://www.harrods.com/brand/royal-collection-trust/coat-of-arms-collection?viewall=yes
http://www.heraldryandcrests.com
http://www.allfamilycrests.com
http://www.thetreemaker.com/coat-of-arms-search.html
http://www.coadb.com/
http://www.fleurdelis.com/nofamilycrest.htm <- found another link that mirrors what Robert said.
I have a few more that I want to vet out. I'll post them here tomorrow when I see that they have a decent number of crests in their collection. Some of these sites even lets you buy them for a decent price.
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