Greg, vine maple seems to be about everywhere on the westside of the Cascades. Walking down a powerline maintenance road, for example, can be a good place to find some, and along forest service roads, too. But in denser forest it seems to grow a little straighter, probably because it is struggling to reach the sunshine and so it grows up, up and up instead of being lazy and squirreling around like it will in more open areas.
You want to cut it yourself so that you know what side was facing up because that is the tension side and should become the back of the bow. You can split it on site by beginning on the top side (right down near the ground) and cutting, keeping your saw horizontal, only half way through the wood; and then reaching up as high as you can on the shrub and pulling downward. It will start to split and the split will run right up it as you pull; move your hands further up it as it gets closer to the ground. Right then you will know if that particular shrub has a tendency to propeller twist or not. If it doesn't twist cut it off a foot longer than your desired bow length, seal the ends and cut some more. [If the sap is still flowing, within about an hour the bark will peel easily, too, saving you a bunch of scraping work later on.] IF it does twist, some people say you should go to another valley and try again, because they feel that twisting is a genetic trait and so many of the other nearby shrubs may have the same tendency.
Don't get caught cutting yew wood on public property without a permit!