Michaels and JoAnns are the two best that we've found. We also shop antique galleries, flea markets (risky but so far no infestations

) and garage sales.
For the fall stuff, local produce markets have everything. Keep in mind that here in Florida, we don't have to worry about snow so stuff gets put out and is easy to find at roadside stands.
For flags/4th of July items, try a flag store. Yup, they really do exist. For Xmas stuff, you guessed it, try a Xmas store.
Stuff that is always good: silk flower arrangements, garden pots, boxes of any size & shape.
Jim