So there are two sites I found where you can make your own things.
One is called Zazzle (where I make phone covers [for fun]), with this you can create phone covers and sell them or even purchase them yourself! (Thought they are really expensive, it's kinda cool!)
Here's one I made:

Cool! right? ( zazzle.com.au if you want to try it)

The other one is spread shirt you can make so much stuff here e.g.
Hats
Bags
T-shirts
etc, etc
They also have phone covers as well! (The only downside is that they don't have all types of phones)

Here are some things I made:

That's a S.T bandana ⬆️

A pillow ⬆️

An apron (if you can't read the writing: it says; where's the food bro?) ⬆️

And a t-shirt ⬆️

They have heaps more things but you get the idea. ( spreadshirt.com.au if you want to try it out!)

See ya!