As people have said, Playbillstore.com has quite a bit. (You can search by specific show). Plus most shows have an online store/merchandise tab on their website.
Sometimes you can find stuff on eBay but it's kinda hit or miss. Depending on what it is/when you find it it can be either way cheaper than buying it brand new, or else crazy expensive...
It's not official Broadway merch, but Redbubble has a TON of Broadway-related designs/lyrics/art/etc that you can get on practically anything. (T-shirts, mugs, wall art, stationery, stickers, you name it).
you found your heart but left a part of you behind <3