I bought mine from Brownels in 1986. I'm sure they still have them. You can get just the 'upper' or the base which fits round barrels as well. I paid $96.00 just for the 'upper' in 86. The prices have not kept up with inflation.

http://www.brownells.com/rifle-parts/sights/rear-sights/classic-express-sight-prod9644.aspx

This is the one I bought. They are now ONLY $155.00 for both pieces, top and bottom.

661-103-830WB
P3-XL Classic Express Sight

Mfr Part: R202XLP3XL