Each Saint is associated with particular symbols or causes and by checking in a picture book of Saints, or doing an on-line search for “saints by cause” or “patron saints” you can easily find way more information than you probably need or want.
Chances are really good that you could simply use toys you already have around the house to pull together a great Saint costume.
Do you have any soldier equipment (plastic swords, helmet, etc.)? Try St. George, St. Joan of Arc, or St. Martin of Tours.
A crown and jewels? St. Elizabeth of Hungary, St. Elizabeth of Portugal, or St. Louis of France.
Carpenter toys? St. Joseph, St. Thomas the Apostle, or Noah
A doll? St. Ann, mother of Mary or The Blessed Virgin Mary
Doctor toys? St. Gianna, St. Cosmas or Damian (a great choice for two brothers or friends who want to work together), St. Luke
Stuffed animals? Noah (two of any animal), St. Francis, Jesus, the Good Shepherd, St. Dominic (typically shown with a dog), St. Hubert (deer), St. Agnes (lamb)