Under USCF rule 8F7:, it is acceptable to place a rook upside-down if a spare queen is not available.
If the desired piece is not available to replace a promoted pawn, the player may stop both clocks in order to locate that piece and place it on the board. A player who cannot quickly find such a piece may request the assistance of the director. It is common practice, however, to play using an upside-down rook for a second queen. In the absence of the player’s announcement to the contrary, an upside-down rook shall be considered a queen.
Under FIDE rules, you can't use an upside-down rook as a spare queen, but you can find the answer hidden in the section about the clocks:
6.11.2 A player may stop the chessclock only in order to seek the arbiter’s assistance, for example when promotion has taken place and the piece required is not available.