The cheapest shops that carry the Supercard DS ONEi are all Chinese shops. They are more or less all carrying it at the same price, varying from $21.50 to $24.
gamekool.com has it for $21.50
ndscardsale.com has it for $21.80
0shippingzone.com has it for $22.95
shoptemp.com has it for $23.00...