Then they are not following church guidelines for sacred vessels.
Who is selling them?
I gave you a link to them on e-bay. You can google “monstrance for sale” and get links to various places. Most decent sized towns have religion supply stores (at yellowpages.com “religious goods” is even an auto complete option you get after typing “reli”) where you can get all kinds of stuff like vestments and portable confession kits. Some might want to see some proof you’re actually a leader of the appropriate religion, especially for the more sacred stuff, but there’s always going to be somebody that doesn’t care. And remember a lot of stuff doesn’t become sacred until it’s brought to a church and had a ceremony performed, until then it’s just decoration.