Bruce most of the better kit or plans producers have good engineering sense and will call out a particular dimension, weight, and grain for the various structures....so yes you need a selection of lumber for scratch builds or repairs....
years ago at a local RC swap meet I bought a Giant Douglas DC3 kit....I think TopFlite...Sunday close down owner wanted it gone and accepted my $25 offer....82" WS multi engine model....was a lot of ply, bass, and various weight balsa....lots of sticks, flat sheet, and tapered or moulded trailing or leading edges
My local Home Depot and Lowes do not carry the typical balsa display but we have a upstart called Home Base that does have a selection similar to Hobby Lobby
send me a PM with address...I am one more stroke away from all stash ending up in same dirt hole as my carcas....this last few months slowly liquidating.....add a engine , fueltank, kit, or building supply wish list to your note