The best stencils come from Cutting Edge Stencils or Royal Design Stencils. They both give videos on how to do it.
Google on Amazon. They have a wide variety. I just looked at them.
Cej, you can find them at Fabric Stores/Craft Stores, probably Target also...You can also check online...JMO
My favorite place is Cutting Edge Stencils. They have a great selection! https://www.cuttingedgestencils.com/
Fabric store, craft store, hobby store, art store, Wal-Mart, Target, Kmart, etc. Plus lots of on line retailers and it goes without saying, Amazon.
You can get stencils at Michaels and Joann for sure. The greater variety can be found online stores on sites such as Etsy.com and Amazon.
What kind of stencils are you looking for? What will you be using the stencils to draw? For painting a wall design, for fabric, etc.? You may be inspired by going to an art store like Michaels, or in the paint section of your favourite hardware store.
Some Walmarts have quite large craft sections, and although selection varies depending on location, I have seen quality brand name products.