Have you got a liquid nitrogen storage facility in the University/Department?

It may be easier and also less cumbersome than renting a new -80

you can try to transfer freezing container with vials to −20°C freezer and leave them overnight. Then place vials to liquid nitrogen tank. But the viability of cells probably will be lower than if you use -80°C freezer as an intermediate step.

If you are simply looking for a method to slowly freeze cells before adding them to liquid nitrogen during a cryopreservation protocol, you could try an acetone-dry ice cooling bath. This maintains a temperature of ~ -78°C. If your vials are in a "Mr frosty", "Cool Cell" or equivalent, their temperature reduces at ~1°C/min and so you would only need to incubate them for around 120 minutes before transferring to liquid nitrogen storage. 

I haven't done this myself unfortunately so I do not have an in depth protocol to send you but here is a brief youtube video, you will be able to find more with a thorough internet search. https://www.youtube.com/watch?v=l9v1K5jF_C0

I hope this helps,

Sam

Thanks everyone for the answer. We have liquid nitrogen and -20 freezer. I am using cool cell. Perhaps I could try to freeze one or two vials and test the methods mentioned. I have also a link on using dry ice which is -78 as alternative to -80 freezer.

www.biocision.com/uploads/docs/CoolCell_DryIceLocker.pdf

Thanks Maelinda. For now, we are still figuring out what is the best way to store some cell lines without -80 freezer. We do have collaboration with Malaysian Pepper Board and they told that we could use their -80 freezer for the time being. I guess that is the only option because using dry ice is quite expensive as the vendor sell it at minimum of 20kg/order.