That would be a great spot for extra storage Dani. Here's a link with some ideas for you
And one more link, take a look at the 3rd option on this site, look at all that storage room! Shallow yes, but a great place to hold lots of small items for crafting
Here's a great craft room with information on how she made the shallow shelving unit behind her door: https://www.cindyderosier.com/2012/09/ask-cindy-scraproom-tour.html
Measure the space to see how wide the shelving can be then you can build them out of wood. I would make them "L" shaped and screw them into the studs on the support. This is the shape I was thinking you can make them. Just make them to the dimensions you need.
you could check the thrift stores for hanging shelves that you may just need to paint and hang them behind the door- I have a large book case with doors on it behind my door in my craft room
I would not recommend that you spend time searching thrift stores for a shelving unit. IMO, it is unlikely that you will find one that would be the exact size to fit behind your door and the right style to meet your particular storage needs.
It would be much faster and less frustrating to build your own.
Adjust measurements for your project
Here is how to create some simple shelving: https://www.youtube.com/watch?v=2Z9bY1tmDCk
Make wooden Wedge Brackets to hold them up. Buy an over the door system. I have one with baskets meant for bathrooms or buy over door hooks and string your shelves up and secure with a Bungie - maybe?
Do you want it on the back of the door or on the wall behind the door?
You can also hang narrow baskets on the back of the door or the wall behind the door. Command hooks are a temporary method to try out the baskets. Use what you have or take a quick trip to the dollar store. Another method is to build a simple frame using wood that fits in the space. You can add shelfs or baskets. Good luck!
You could put up a pegboard and then hang shelves, baskets or even mason jars from it.
This post shows how you can use pegboard to hang shelves, baskets and mason jars. etc: https://thediymommy.com/a-craft-room-office-pegboard-gallery-wall/
This video shows how to hang DIY wooden shelves from pegboard:https://youtu.be/maGhsJy32aI
You could try this: https://www.aratariathome.com/blog/how-to-install-a-diy-floating-shelf
Can we see a pic of the space you have?
Would you share a picture of the area?
They do sell very narrow shelves for spice racks, I would investigate that if you're looking for ready made.
Here's a post that might help - https://thediyplan.com/diy-back-of-door-shelf/
Is the door solid or is it hollow core? I'd seek out something that hangs from the top of the door so as to not damage the door in any way, unless that's not a big deal. Great way to make use of unused space.