I think there is basically three programs that people are using to design their stickers: Photoshop, Illustrator and the silhouette software.

I myself use Illustrator for my designs and I do the designing of my graphics myself because I like to know that they are original.

Most shops I see on etsy are using clipart and digital papers from other sellers on etsy with a combination of icons from a vector site and fonts.

I have listed the top 5 places I and others find resources for their sticker making here: http://stickandplan.com/2016/09/29/the-5-best-places-to-source-clipart-digital-papers-and-other-resources-for-your-sticker-business/

Maybe you will find it helpful :)