You can usually recognise the shops selling sims because they'll have a board with available numbers posted. Note that prices vary depending on the luckiness of the number.
And, unlucky numbers are often offloaded below cost (the game is, if you buy 1,000 SIMs at a cost of Y50,000, you should be able to resell 10 of them @ Y1,000/each, 20 for Y500, 50 for Y150... take whatever you can get for the lemons).