You can try eBay. I was able to find Mantis Q batteries on there, however they're GIFI batteries. I've got two of these and they worked well, but battery life isn't as long as the original batteries. Here's a link if you're interested.
I bought one of the GiFis on Ebay and it worked well on a maiden flight over the weekend. I got 17 minutes of flight out of it, bringing it back for landing at around 35%. It doesn't sound like much but since my used OEM Yuneec battery only gets me 10-15 min that's a pretty good improvement.
I modified the old battery and took out the dead battery cells. I saved the connector and modified the case. It's not as beautiful as the original or $70 replacements but who cares when its up and flying for $20.
You need a good fine tipped soldering iron. I bought the Hakko brand at Homedepot online. If you have a shrink tube to cover the solder joints the better. I only used the JST-XHR to connect to the original Yuneec connector that I saved.
I tried it once and it flew but I still have to check the the auto landing.