According to Halfbrick's Rolling inventory FAQ
How does it work?
All jetpacks and outfits, both new and old, are now on a giant loop. There will always be a set number of items available for purchase and these will be rotated on a weekly basis.
As new items are added, others will be cycled out. These could be completely new items or fan-favourites returning to the game. The process continues until we’re back at the beginning.
What happens if I miss an item? Is it gone for good?
Nope! It’s important to note that no items have been permanently removed from the game. Items which are currently missing will eventually return as the cycle progresses.
What about the Rainbow Jetpack? I can’t get the Hippy achievement!
Don’t worry, it will definitely be back. Keep an eye on our Facebook and Twitter pages so you don’t miss out!
If it isn't available for purchase in The Stash right now (it will be in the top of the list with items available for purchase), then it has likely been rotated out for a while. I suggest checking back once every so often to see if it is there and maybe keep some coins reserved for the purchase so that you won't miss it next time.
According to the FAQ, "It takes approximately 22 weeks for a full loop to complete." so you might have to wait a while, but that means you'll have plenty of time to collect coins.