I think it's decent for the couple to pay any expenses that wouldn't be incurred by other wedding guests, so probably just any special clothes really, unless it was something like bridesmaids being expected to stay in a hotel when they wouldn't have stayed over otherwise, then I think the couple should offer to pay.
When my daughters were flower girls for my sister, I offered to buy their dresses as I would have bought them dresses anyway if they weren't flower girls and it was something my sister and I chose together and they got to keep them. My sister paid for my dress and shoes (bridesmaid), but if her budget had been tight I would have been happy to contribute as I kept them afterwards and they can be worn again. I wouldn't consider it rude to ask a bridesmaid to pay for either the fabric or the dressmaker if dresses were being made specially and could be used again by the bridesmaid, but it depends on individual circumstances.
For page boys and ushers, if they are asked to wear specific suits when as a normal guest they'd just wear a normal suit, I'd expect the couple to pay for suit hire. If they were asked to wear a normal suit but certain tie or something, I'd only expect the couple to pay for the tie.