While there may be places on the Internet where you could download the AnyConnect client, the official way to download the AnyConnect for Windows client is from the Cisco web site downloads pages. And to download from the Cisco downloads pages requires having a CCO ID which has download privileges.
If you are the original purchaser of the ASA5505 there should have been a copy of the AnyConnect client that shipped with the ASA5505. If you can find that copy of the client then you do not need the Cisco download pages.
It is a bit of a long shot but you might contact Cisco TAC and ask if there is a way that you could get access to download the AnyConnect client for Windows.
HTH
Rick
HTH
Rick