I have been asked about this a few times in the last few weeks about it.
So in no specific order, here are your options.
1) Use someone else's pc that has the programs running.
2) Download it all from IBM if you are entitled.
3) Call your local Business Partner and get it via FTP
4) Go here and download it from IBM, the 100% FREE Designer client that is.
5) Go here and download the 90 day trial Admin and Notes client code.
6) If you do not have any of these options, then you want to print and keep this page about Lotuscript to do it for you. Thanks to the SearchDomino site for posting it.
With this LotusScript code, you can sign a Notes/Domino database without using Lotus Notes Administrator. Just create an agent with the LotusScript code snippet below and then run it. Right click on Run in your client, or SignAgent?openagent through your Web browser to sign the Notes/Domino database.
Dim session As New Notessession
Dim db As Notesdatabase
Set db = session.Currentdatabase
7) Nomad, run Notes on a USB! It will run designer and admin clients, keep it handy just for this purpose.
8) RDP to a machine with the admin client, copy the file over or put it on a server and get it done.
Thanks for the added items from the comments
9) Another option is to use the free utility "Simple Signer", developed by Tony Austin in Australia. http://www.asiapac.com.au/Simple_Signer_Download.htm
10) Copy designer.exe to your program folder, it worked prior to R8.