The following is what you might be looking for:
K4633-40038 GN RPO 38
K4633-41327 GN Vista Dome 1327
K4633-41373 GN Sleeper 1373 Santiam Pass
K4633-41390 GN Great Dome 1390 Glacier View
Note that the 41390 is a full-length dome as opposed to the shorter vista domes.
As for availability; train shows, sales lists such as OGR, ebay are probably your only choices.
The K4633B set mentioned above contains the following cars:
K-4633B,GN,Empire Builder, ,6-pack
K4633-40263,GN,263,Baggage, , , ,
K4633-41190,GN, ,Observation,Choteau Coulee, , ,
K4633-41252,GN, ,Diner,Lake Ellen Wilson, , ,
K4633-41260,GN, ,Sleeper,Skyhomish River, , ,
K4633-41330,GN,330,Vista Dome, , , ,
K4633-41390,GN,390,Full Dome, , , ,