I have Nikon P500 but not able to find original Battery in Market. Is there any other way (external power/other battery) to P500?

According to page 213 of the User Manual, the Nikon P500 uses a EN-EL5 battery.

The Nikon EP-62A AC adapter is compatible with the P500, and replaces the battery in the camera with a battery-shaped adapter that can be plugged into an external power source.

Watson makes a replacement battery for the EN-EL5. B&H Photo carries this battery for $15.

Though Nikon no longer markets the EN-EL5 battery, there are still plenty of third party manufacturers that do offer compatible EN-EL5 replacements.

Here in the United States, they're offered by retailers such as B&H. On amazon.com there are a wide variety of third party EN-EL5 batteries and chargers available.

I'm a Canon shooter whose last several camera bodies all use the near ubiquitous Canon LP-E6 battery. Even though Canon still makes and sells the latest version of this battery, the LP-E6NH, I mostly use third party LP-E6 batteries. The only genuine Canon batteries I've ever owned are the ones that came with each camera. I've found that the third party batteries from reputable names bought from reputable sellers tend to perform almost or just as well as the Canon versions do at a small fraction of the price.

STK, Watson, PAWA, and Maximal Power are the ones I've used the most. Though I will say that Maximal Power seems to have recently dropped a bit in terms of longevity. These same brands, as well as others, offer your Nikon EN-EL5 replacement batteries as well as my Canon LP-E6 replacement batteries.

Searching for "Nikon EN-EL5" from your location should return a list of available batteries in your area.

Buy from a reputable seller and you should be OK.