Sierra trading post does not carry the mainline as far as I know.
its the Sterling, half canvas, high arm holes, slimmer fitting, flat front, etc. You may like this.
Finding a Hickey Freeman Mainline in a basic Navy solid or stripe, Charcoal solid or stripe in an average size for under $500 doesn't happen everyday in every neighborhood.
If and when you do see one grab it, and then come ask questions, not ask questions hoping it will be there when you go back.