I love collecting things that I can actually use, and since the kitchen is where most of my time is spent - mixing bowl collections became a natural fit. I tend to look for vintage sets, with little to no wear on them. Some sets are impossible to find new, like a set of Yellow-ware or original Fiesta-ware.
I tend also to not only using my bowls for decoration and actually mixing of ingredients, yet also for serving foods or using floral tape on the top to create a temporary flower frog to hold a bouquet of flowers. Vintage brands I look for are Fire King, Fiesta ware, Texas-Ware, Pyrex and Yellow-ware. I also love the glass bowls from vintage mixers.
There is a brand called Mason Cash, an English company that been around for over 100 years that moved their manufacturing facility over to Portugal in the last decade - I look for these work bowls stamped 'Made in England' on the bottom since they have become a rarity, mixing the interior colors.
I have found some unique mixing bowls sets at better kitchen stores and home décor retailers, where they are obviously new - yet interesting materials like bamboo, or imported from another part of the world such as France. A current brand that I watch and collect is Emile Henry, a French brand that uses a Burgundy clay as the body - why this is of interest is because it makes it oven safe. The utilitarian aspect of that is appealing to me.
Vintage and current manufacturers can be found on websites such as eBay or Etsy, scanning antique stores or tag sales.