I've seen two types of fitting for this. One is a sweat adaptor where one end is a female sweat and the other end is a barbed insert fitting for the pex. The female end is sweated to the copper.
The second is similar except that the female end isn't sweated to the cooper...it attaches with a compression joint...just wrenches needed. Try a plumbing supply store.

Thanks Cool.

I've looked at the packaging for the Compression-type fittings and they state that they are not intended to be used with hidden plumbing. I'm not sure if that's a Code issue or a wise warning from the manufacturer, but it should be considered carefully depending upon where you intend to use them...

Cash Acme, based in Alabama, offers SharkBite Push-Fittings that would be ideal for your application. SharkBite fittings can be connected to cPVC, PEX, and Copper in any combination with no tools. It is certified for underground and behind-the-wall applications. You can pressure test immediately with no wait time. Also it is certified up to 200F and 200psi.

Here is a link that might be useful: SharkBite Product Info

shark bite, hands down