I buy mine directly from the source, XS Sight Systems. I do have one of their Picatinny rails for mounting a scout scope, but it's not currently on any gun. They now make rear sights that will mount to the Picatinny rail, so you can have a scope and backup peeps on the same rifle. The rail mounts to existing holes in the receiver and the rear sight dovetail.
I use their standard sight packages that mount directly to the receiver, without the rail. They all mount to existing scope mount holes; no drilling and tapping required. Here is their website:
XS Sight Systems - Rifle Sights