Now that you know what structures you're trying to find the answer is simple. Keep looking.

It took me a long time to find burial chambers, the Black Forests that I found early on in my first map had very few near my starting area. Eventually I found plenty of them but I had to explore a lot. Sometimes you'll luck out and find a lot of crypts in the very first swamp you find, usually you won't. Good luck, I hope you find more iron than you need!