Here's the Amazon link. They were relatively easy to install if you find the YouTube videos and they work great. I have a 7,000 lb travel trailer with a tongue weight around 500 lbs and with these inflated, it rides perfectly level.
AIR LIFT 60818 1000 Series Rear Air Spring Kit https://www.amazon.com/dp/B001OMVCIC/ref=cm_sw_r_cp_apa_IV4JAbG89VXB3
If you are trying to take up sag, have you looked into bagging the rear shocks? WE did that with our Previa and it made a world of difference for balance and handling.
We used this kit. Obviously you need the one to match your vehicle...
We tow with a Toyota Sienna. we bagged the rear shocks and it made a huge difference to handling. No more rear sag and way better braking and cornering.
A kit like this does amazing things. You just stuff the red air bags inside the rear shocks and inflate when you are towing something heavy.
$100 set of airbags fixes this problem.
https://www.amazon.com/gp/product/B001OMVCIC/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1
I put these on my ram. Makes a big difference in sag when putting weight on the rear axle.
Not too hard to install.