I have just received some third party props purchased from Amazon. The quality seems to be comparable to the official props, the packaging is good and the install diagram is a nice touch. My Tello flies true. Bargain.
A quick update. My boys and I have had some more fly time, and some more crashes. The props have picked up a couple of nicks which the originals seem to resist more. I think the plastic may be a bit harder. They haven't broken and the Tello is still flying. I still think they are worth the money.