PNG – Portable Network Graphic format offers a transparency options. It is a raster graphics file format that support lossless data. Let us see a website were we can find this free PNG images.
PNGDB has 22 million Free Transparent PNG Images for personal use. First you need to sign-in to download the images. You can browse images through color and by category.
They also provide premium plans to use the images for commercial use. For this go to the premium option and select lifetime or monthly plan if you can afford for that price. For free plan you can use limited options from the website.