The following flowers usually have the best availability:
Roses, Gerber Daisies, Lillies, Alstroemeria, Solidago, Lysianthus, Birds of paradise, Dendrobium orchids (need to be ordered ahead of time), green hydrangeas (need to be ordered ahead of time), iris, carnations, daisies, gladiolas (need to be ordered ahead of time), liatris, star of bethlehem, button mums & spider moms (creme and yellow available), white calla lillies (alcatraz), larkspur, babies breath, delphinium (can be seasonal), asters.
Orchids such as cymbidiums have to be pre ordered at least one month before and they are shipped in from either Mexico City or The Netherlands. They have a fee of approximetely $20 a flower.
Colored calla lillies are also available,but need to be preordered as well. They are a bit more expensive, i am not sure of the exact fee, but quite reasonable.
I believe you could even get tulips, however I never suggest using these. Our experience is that they do not withstand the heath that well.
Please dont hesitate to contact me if there is anything I can help with!