You'll find links to the official merchandise on a large majority of the shows' official sites these days, either hosted directly on that page or linking to the site the merch company for the show has set up.
Harry Potter and the Cursed Child has closed their online merch store. I went on the site last week to look for the window card and there was a notice. It appears that they have totally taken it down as I don't even see a link for it on the site now.
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
THANK YOU!! I was so sad that I wasn't able to get a Once On This Island merchandise item but because of these links, I was able to find it!! Thank you!!!