Is there any way to preorder the 29.00 battery for my iphone 6

You would have to contact Apple about that.

Here is the latest info -

We’ve always wanted our customers to be able to use their iPhones as long as possible. We’re proud that Apple products are known for their durability, and for holding their value longer than our competitors’ devices.

To address our customers’ concerns, to recognize their loyalty and to regain the trust of anyone who may have doubted Apple’s intentions, we’ve decided to take the following steps:

• Apple is reducing the price of an out-of-warranty iPhone battery replacement by $50 — from $79 to $29 — for anyone with an iPhone 6 or later whose battery needs to be replaced, starting in late January and available worldwide through December 2018. Details will be provided soon on apple.com.
• Early in 2018, we will issue an iOS software update with new features that give users more visibility into the health of their iPhone’s battery, so they can see for themselves if its condition is affecting performance.
• As always, our team is working on ways to make the user experience even better, including improving how we manage performance and avoid unexpected shutdowns as batteries age.

At Apple, our customers’ trust means everything to us. We will never stop working to earn and maintain it. We are able to do the work we love only because of your faith and support — and we will never forget that or take it for granted.