Go to the link below and click on the XM Satellite Weather icon.
http://www.garmin.com/products/gns430/
This will explain how the NEXRAD weather radar images can be uplinked to your Garmin.
If you go to the XM Satellite Weather site, you can see that it's available for many other nav/EFB devices as well. But since most people are going with Garmin today, I sent you that link.
IMO this uplink of NEXRAD images is by far the most bang for the buck...most of the utility and features of weather radar for a fraction of the cost. In fact the only drawback/difference between on-board radar and this product is that the radar "picture" only updates every few minutes. At helicopter speeds, that shouldn't be much of a problem.
If you absolutely have to have real-time information, then bite the bullet and buy a weather radar, because in my experience the Stormscope-type devices don't present much useful information for weather avoidance in VFR helicopter applications.