Barrister bookcases are nice because they keep everything encased and look classy. They are common and cheap used online, just be sure to check the dimensions as only some will actually fit your pickelhaubes.
On flee markets and vintage sales.
I regret having baught a 1000 euro ikea display (book) case.
While I had 2 antique display (book) cases, in oak for 600 euro....
The only thing is it took me some time to find the last one's and had to go and fetch them.
The ikea stuff was ordered from home and delivered at home....
The only company I've found available in the States that make very nice Curio Cabinets is Howard Miller. Give them a google and check out their Curio selection. Not cheap but very good quality, also good to know is they ship assembled which was a problem for me since my War Room is upstairs.
I also collect WW2 helmets. I have a few of these in display boxes used for NFL/CFL football helmets. They look great with a mirror on the back. Unfortunately (to my surprise), Pickelhaubes do not fit given their height unless you let them sit on their brims....not a good idea. They work with WW2 Stahlhelm, but NOT the haubes. Steer clear of these types of display cases/boxes. Happy hunting.
The link that I sent are for acrylic cases and they are on the platform aliexpress which are all sellers in China. They can tailor made the cases even just for one piece and they ship worldwide. When u key in your address at the set up stage, they will calculate shipping at the checkout and u can abort the whole deal if you are not agreeable to the shipping charges