If I'm not mistaken, a player running out of balls during a round can obtain more balls from "any source" (even from their opponent!).....provided that he or she doesn't unduly delay play in the process.
From a practical standpoint, that basically leaves only your opponent(s) as a source, unless you happen to lose your last ball right beside the pro-shop and can quickly dash in and pick up another dozen?
I'm guessing that if this happened on the PGA, there would be a caddy position available the next week...lol
I agree with the OP though...if I have numerous diferent varieties of balls available to me, why does it always default to the starter balls if I run out of one particular type?