If it has both the Stevens and Savage names on it, it was made between 1948 and 1991. Any gunsmith should be able to find parts for it, or you can try Numrich Gunparts.
The exact value of a Stevens 12 gauge model g20a is actually dependent on a couple things. Some of these things would be the age and condition.
what is the value
How old US a Stevens savage model 530a 16 gauge
Impossible to give age without sn. 50-100 USD
The Model 620 was produced from 1926-1945. sales@countrygunsmith.net
That depends on the model number, age, and condition. All of those will significantly affect the value.
it was made in or around 1972
I would say it's around 75-65 used.
Late 1930's, value up to $100. sales@countrygunsmith.net
These where made from 1955-1971. If in 90% condition about $130.
A Western Field 30 is actually made by Stevens as the model 520. Serial number dating was never kept on the Stevens model 520. They were made by Stevens between 1912 and 1932. Value is typically a shooting value between $100-200 depending on condition.
Your Stevens model 9478 single barrel shotgun was made from 1987-1988.The value is between 60-85 dollars in the condition that you describe.