Well, besides chiif, my favorite 39mm caps and other 39mm stuff (step-ups, rear caps, filters) are from heavystar on eBay.
I like them because they don't stick out like the original Leica 39mm caps which I don't like. These snap-ons have a low profile and look like filters instead - e.g. like you can slide out the built-in hood on a 50mm Summicron without the cap blocking it. Be sure to buy spares.
http://cgi.ebay.com/New-2-x-39mm-Sn...Accessories?hash=item53de97b58b#ht_500wt_1182
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