You buy it straight from Yuneec, they are the only ones, to my knowledge, that sell it direct at a reasonable price point.
Expect to pay $9.99 each and I'd suggest you buy 2 or 3 since the shipping cost wont change. Part# YUNMK58101.
You might find one available from another Vendor but expect to pay $15.00+.
You can call Yuneec Tech Support, 1-855-284-8888 and they can connect you to the Sales Dept. or give you the phone number for Sales.
I see this is your First post and you just joined today, so Welcome to the Site!
It only took me a couple of days to break mine. I ordered a couple from Yuneec and it only took 3 days from CA to TX using USPS. It cost me about 22.00 shipped for two. I was glad to find out that they were not charging like $30.00 bucks from them. I am sure other companies would.