There are numerous online sources and parts dealers at gun shows. The largest are Numrich Gun Parts and Gunbroker, but if you web search for that model you should find several different sources.
I purchased a new Harrington & Richardson 12 gauge model 088 from K-Mart in 1980.
36GA
model 158 12 gauge shotgun
Less than $100.
$100-$125.
50-150 USD
$50-$125, depending on condition.
50-100 USD
100 USD give or take
I just bought one for 70$. Light surface rust, cleaned off easy.
About $70, depending on condition.
They sell new for $125.00