Hi, my name is Jay, I'm a long-time craigslist Mac tech, specializing in hardware and software, networking, repairs, etc.
I have a fairly extensive set of new in box MacBook Air and MacBook Pro battery replacement kits. They are essentially just like the Apple batteries; nothing is different between the Apple batteries and the ones that I have, but they are third party batteries.
The plus side to that is that I can offer the batteries at a competitive price, and I do either direct sales or on-site services for the installation of these batteries. The kit to remove the old battery is included with each kit battery item, as well as to install the new battery, so you have the option of doing it yourself, for $45, or I can meet with you somewhere and install the battery for you, for an additional $25.
I also have access to various other laptop batteries - Windows devices, for example. If you'd care to, you can email me the model laptop you are using, and I can check my resource to see if a replacement battery is available for me to sell to you, or to install for you.
Please send an email with the model MacBook Air or MacBook Pro version you have - I have batteries available from about 2012 through the present day, covering all MacBook Air and MacBook Pro models from back then until now, I might need a couple or a few days to place an order and to receive the product, in some cases, but I do have plenty of batteries on hand, as it is.