Brownells has scopeblocks for some rifles.
Best bet is gunshows from a sight dealer/collector.
You will need the hole spacing as well as the difference in height for the barrel and receiver block (hint: 1/2 the difference between the barrel diameter and receiver diameter. Works on the Stevens since the barrel is not tapered.)
Make sure you get the Unertl ones and not the Posa ones, they do NOT interchange. For a good description of the differences, see this site.
http://www.alexweb.net/unertl/ He has a page describing the differences.
It always pays to take dial calipers to shows for these types of things.
Good luck.