Can you get a different rear sight for a Lyman great plains rifle, something similar to say a TC Hawkens, I really dont like the buckhorn sight
Another question will a rear sight from a 1853 Enfield work on a 1861 Springfield?

I also have no use for a buckhorn, really can't imagine why they were ever made, but I don't think much of the T/C rear sight either. The issue with them is the elevation screw detentes depend on spring pressure to hold the leaf up against the screw. If the leaf is accidentally depressed, as can easily happen in a gun case while being transported, then the elevation screw turns freely and can vibrate up or down, often to an extreme loss of zero. I prefer the traditional fixed sights, takes a little effort to zero but once done it stays done. Your barrel is cut with a metric dovetail just a bit smaller than the 3/8" dovetail standard in the U.S. but it is no great problem to file a 3/8" sight base to fit. I'd suggest you check out Track of The Wolf, they have a really great online catalog with good photos and general dimensions of the sights. Measure your current sight from the bottom of the dovetail to the point where you normally rest the front bead and Track will probably have something to fit. If your present front sight is a straight, flat top blade you can get a shorter rear sight and file the front down to zero in. If your present front is a bead and you wish to keep it you will need to get a slightly taller rear sight and file that down to zero. Or you could get an elevation adjustable rear sight with positive step adjustments. If you really must have a T/C rear sight they are available also but will require drilling and tapping two screw holes in the barrel. At any rate, Track of The Wolf has a good selection and it costs nothing to browse their site.

When I bought my Great Plains rifle new, two different rear sights were included. One primative fixed, and one modern adjustable. I would contact Lyman and ask for the modern one.

I may be wrong, but I seem to recall that both the fixed and adjustable are buckhorn style, but of course a hacksaw and file can fix that.

Lyman has a hunting sight set, with a white bead front sight and folding rear. The sights are the same as they put on the Deerstalker. I have been using them for 16 years now, and really like them.

Yes the Lyman Great Plains Rifle's dovetails are metric. The Lyman folding rear sight does not fit the rear dovetail on the Great Plains rifle. You have to peen over the guns dovetail to get that sight to fit (thus beating up the gun). The dovetail on this gun is larger than the dove tail on the folding sights.

I ended up using this sight for the rear -->
http://www.trackofthewolf.com/(S(z1rnsjfjrzxduqfpwo55l245))/categories/PARTDETAIL.ASPX?CATID=14&SUBID=167&STYLEID=767&PARTNUM=RS-DRU-CT

And using this sight for the front -->
http://www.trackofthewolf.com/(S(z1rnsjfjrzxduqfpwo55l245))/categories/PARTDETAIL.ASPX?CATID=14&SUBID=167&STYLEID=762&PARTNUM=FS-1803-I

This sight would probably have required less fitting for the front -->
http://www.trackofthewolf.com/(S(ml4fheb5k5m0uruhuxi0et45))/categories/partDetail.aspx?catId=14&subId=167&styleId=762&partNum=FS-G-77-I

You will have to do some fitting for these types of primitive sights. I ended up deepening the rear sights slot making it rectangular with a very small file for a slot and post type sight picture. I used a brass front sight originally but I dropped the gun and the sight moved so I switched too steel one for the front. The Brass sights are very soft.