Icons for iOS (and watchOS) Development

I am looking for a good source of custom icons for a business iOS app we are currently developing. Our app is currently targeted for iPad, although an iPhone version will follow soon. Furthermore, we expect to our app working with watchOS as well.

It is possible that some artist, product or web site has art that we can use for our in app icons, and that would be the ideal solution to just purchase existing icon art, but we might need to go down the custom art road as well.

To date, I have only used existing art provided by my corporate clients and this will be the first time I need to obtain my own icon art.

If you can point to some good providers of icon libraries and/or artists that are familiar with this environment, your assistance would be greatly appreciated.

Thanks in advance for your help.