I don't think that's possible without scripting.
"Location Available" (id: locationquantityavailable) cannot be chosen as a source (from the item record) for custom column fields, maybe because it wouldn't know which location's value to source.
Even if it were possible using a formula field, the value would then only be displayed in view mode after saving the transaction, not while entering lines.
Your best bet would be a client script using the postSourcing event that fires on changes of the item column.
You could then perform an item search filtered to the current line item & your alternate location, and populate your custom column field with the result from a single search column ('locationquantityavailable').