Most front sights have to be silver brazed ("silver soldered") on.
This means the barrel will have to be refinished since the red heat needed to flow the braze will ruin the finish.
Since the "hot" item in shotgun sights are ghost ring receiver rear with rifle front, that's what's listed the most.
Since you didn't specify whether you wanted barrel-mount sights, that's what I listed below.
What you can do is mount something like a William's rear with a front ramp model:
http://www.brownells.com/aspx/NS/store/productdetail.aspx?p=16358&s=36570
The William's rear is attached with screws.
http://www.brownells.com/aspx/NS/store/productdetail.aspx?p=16148&s=36208
An excellent option is to buy and install a Remington factory rifle sight set.
These are the best barrel-mount shotgun rifle sights made.
These are available from Brownell's and direct from Remington.
On these, both the front and rear sight bases are brazed on.
Last, a really good option is to buy a spare Mossberg barrel that's equipped with rifle sights. These can be found for as little as $100.00 new.
Or you could contact Mossberg and buy a set of their rifle sights, and either have them installed locally, or send the barrel to Mossberg for installation.
http://www.mossberg.com/