The cheapest best GR rear sight is the one made by Mossberg. Also, your receiver may be drilled and tapped for it. It's ugly, but it works well. For more money and a prettier rear sight, you can go with MMC or LPA.
For the front sight you can use the Mossberg, but it has to be silver soldered on the barrel. MMC and LPA also make good front sights that have to be silver soldered. The barrel will have to be refinished.
MMC and LPA also make clamp on front sights. You can get the MMC one with tritium and I think LPA will have a tritium insert for theirs pretty soon.
As with most things, you get what you pay for and usually less.