How can I get/install BIG-IP Edge vpn for Ubuntu 18.10? Or can I use any alterantive?
You can download it only after proper registrations (it's free).
Go to http://downloads.f5.com, it will redirect you to login page. There you have a register link. Just register, follow instructions on mail, and then you just have to login to be able to download software.
For BIG-IP client there is a big .iso (APM Clients, +200MB) with all the BIG-IP clients for all platforms. Download it, extract it, find yours and you are ready to go.