You can buy buckets of generic bricks but they're hard to find in toy and department stores. I'm in Australia and gave up trying to find generic stuff in stock here and ordered in Duplo from Amazon for my two year old.
He has airport and train sets (gifts from family), plus bulk bricks, which means he can get a mixture of general building experiences, plus specific things to combine with them. Works well.
He has airport and train sets (gifts from family), plus bulk bricks, which means he can get a mixture of general building experiences, plus specific things to combine with them. Works well.