How to get the Trusted Identity Login Page with the needed parameters to make custom login screen instead of sharepoint Login Page?

hi guys

i have configured trusted identity provider for my public facing internet portal, but i dont want to use the login screen

since i have about 10 site collection which will use this authentication.

is there a class or property that gives me the url ready with the parameters like "wa" and "wtrealm" and the redirect url based on the place the user click the link from.