You didn't say how old your rifles are. Current generation has round rear action, some older ones have flat. The current generation of Stevens 200 (since 2006 & others) takes #46 bases.
Bases can be purchased from Midsouth, Midway, Brownells, Cabelas, Gander Mt., probably even in the very shop where the "gun guy" told you they don't exist, & other sources too numerous to list.
You said "change scopes". Does this mean you already have scopes setting on rings & bases?
Sorry, but your gun store guy is not the" sharpest marbel in the bag". Hope this helps- Good Luck- Good Shooting- Jim