You can pick up a box of 10 Energiser industrial or Procell/procell intense (duracell ) industrial for about a fiver, with long expiry date, or a box of 5 for about £3.50.
I get mine from pstsupplies mainly, sometimes gadgetowl. Both are on ebay and sell genuine batteries.
They are the same as the equivulent retail batteries but, come in box or loose depending how many you buy, rather than retail blister packs. This therefore keeps the price down for trades!