Your florist should be able to help you with the types of flowers that are available – ours looked at the photos I brought and immediately told me "You won’t be able to get those, or those, or those…" She had great little 4 x 6 flashcard books of flowers by color, and we went through those and picked out what I wanted from what would be reasonably available.
My sister actually had flower arrangements, bouquets, bouts, and corsages that all matched for her wedding (lots of red roses). I didn’t want my stuff to match, just to go together on a common theme, which is sort of English Garden (light, airy, unstructured) and in shades of pink and lavender with darker purple accents. We chose the bouquet flowers in pale shades (cream roses with pink edges, pale pink and lavender lisianthus, baby pink roses, pink waxflower). The groom’s bout will be the pink-edged cream roses; the other bouts the baby pink roses; the corsages cream roses with pink waxflower. The centerpieces, which are fairly low, have the pink and lavender lisianthus, lavender roses, baby pink roses, purple lisianthus, the waxflower, lavender astrelomeria, and pink stock (so darker and brighter colors than the bouquet, and no cream). The other arrangements (altar, guestbook table, cake table, piano, etc) are tall vase arrangements or long, low arrangements, and have the lisianthus, the astrelomeria, the stock, and pink, cream, and lavender snapdragons.
My florist worked all this out with us in about an hour, based only on flower photo (the bout that my Fiance wants – he is picky), and my description of the look (English Garden). And I told her that I love lisianthus, and really like stock and snapdragon, and preferred baby and bi-colored roses.
I think that knowing all about flower availability and such is primarily useful if you are DYI’ing your flowers. Otherwise, your florist should be able to help with everything you need to know. The photos I did bring with me, which were a huge help, were of the venue – in particular all the places that I wanted flowers that weren’t centerpieces, and also a photo of how they will set up the head table. That way the florist could advise us as to whether a tall or long, low arrangement would be best, and get an idea of what size the arrangement should be.