REST API to get available node

Well there's not an easy method. The only API that i could think of is,

https://localhost/mgmt/tm/ltm/virtual-address

Get the list of addresses, then work it out with the vip rail subnet & determine what's left.

Then ping that ip to confirm its indeed is free.

Then again do a reverse lookup to confirm if its not reserved for anyother use.

Finally you'll have your free ip.

Every org has their own way of determining their free-ip's. Some use scripts, some use automation tool.

Hope this helps.