Most of those items are made by the children of Hephaestus, cyclopes in the ocean forges, and such...In shield that Percy gets in the movie is in the book, but it's a wrist watch that his half-brother Tyson makes for him (Tyson is a cyclopes). So yeah, it's possible for other demigods to make stuff like that. Children of Hephaestus would likely be better at it than other people though.