If you use the eBay app, many filter options can be "locked" if you enable lockable filters using the customize feature of the filter menu.
If you are using a desktop browser, although some features can be set using the customize menu, item location is not one of those. You can set up a search the way you like it and save it or bookmark it. Then, as long as you start from that saved search, you can continue to use those search options for subsequent searches.
There is a third party browser plug-in called "eBay item location -- my country only" which can be added to most browsers to override eBay's default item location.
Keep in mind that item location information is provided by sellers, and not every seller that advertises an item as shipping from the US actually ships from the US as they should, though.
