http://www.thespeedofwood.com/Mantle.html

Lots of choices or they build off of your picture.

Hi, I saw a specialty site on the internet,just google fireplace mantles. They look pricey but nice, however there are other choices depending on how rustic you want. I built a cabin years ago and the place I went for stair planks and a bar top and a mantle was a saw mill. When logs come in they saw them to your thickness, I got 2" slabs with the "skip peel" edge still on. I asked them to plane them top and bottom but leave the raw edge on one side, that edge is the outside of the log after it goes through the mill. Some time they throw that plank away but they really turn out nice with lots of sanding and lots of coats of spar varnish on them. And the little bits of bark stays on when there is lots of varnish. Second hand furniture stores and auction houses also have great finds. I once found a bed head board that had lots of carving and made a shelf out of it.
Post pictures when your done, we'd love to see it.