http://www.workflow-research.de/Downloads/BPMN/
By far the best one out there seems to be this one..
http://www.workflow-research.de/Downloads/BPMN/Frapu-BPMN_Template(v1.1).zip
I just tried out .. it seems to work with Visio 2007 as well. Though it still looks rather clumsy. But you can take it as a start point and apply styles and other changes to suit your needs.
There are so many others if you search but most have some problems. See this one for example..
http://www.bpmn.org/documents.htm
http://www.bpmn.org/Documents/BPMN%200-9-V2003.vss
I thought coming from bpmn.org this one should be the best. But the different stereotypes for gateways seem to be missing. (OR, XOR, AND)
If you find something better, let me know..
Edit 1
Hey ..
I think I have found a better one..
Go here http://sourceforge.net/projects/bpmnpop/
n download the plugin..BPMN_for_VISIO 0.6
I found the stencil good. There are some macros in the .vst file (template). If you are not sure, just extract the stencil and use it (file named BPMN.vss in the zip file)