Sophia
You could try some easy stuff but timing is everything. My suggestion would be to track down a flower grower at a local farmer's market near you who can probably provide you with everything you could want and more at a reasonable cost, unless you're talking orchids, roses, carnations, etc.
You are not going to find any specific cut flower variety plants in garden centers that you can just buy and put in the ground to grow. For specific colours and variety of plants you would have to purchase seed, grow the baby plants (plugs), provide support if required, provide weather protection if required, fertilizer, water, etc. I hate to say it but it's a little bit more involved than some people think. For April/May flowers, everything should already be started or in the ground growing now. Some suggestons are:-
Yellow - Rocket Snapdragons, Coreopsis, Statice
Royal Blue & Purple - Cornflowers, Hybrid Delphiniums or Belladonna Delphiniums and Larkspur
Bright Pink - Larkspur, snapdragons
Kelly Green - Green Mist (Ammi Visnaga).

There are lots of different flowers that are available in the april/may time frame in the colours you want but you need experience growing them. Godetia, Campanula, Bells of Ireland, Bupleurum, Ranunculus to name a few.
Good luck, hope I've helped.
Cheers Kim
Billabong Fresh Flower Farm

If you're in Zone 8, you might be able to start some things earlier than other folks would. However, I'm going to echo everything that Kim said. If you want to start seeds, you will need somewhere to do so, but check out the Winter Sowing forum. That might be an option for you.

If you are intent on growing your own flowers, talk to your local nursery and see if they will order plugs or 3" annuals for you. They will need to go in the ground fairly quickly in order to be ready by May. Even then you'd be pushing it.

In Zone 8, I'm pretty sure you can direct sow sweet pea seeds in February and March and have flowers by the end of April. You'll need a trellis for them to climb on, but they are beautiful wedding flowers and very hard to find at the florist.

Perennials would need to be started a year ahead of time to produce enough blooms for a wedding, so unless you already have an established perennial garden, that's a no-go.

I recommend looking for a local grower who might be able to custom grow some flowers for you, if growing them yourself is not a viable option.

I'll throw in my 2 cents worth and agree with Kim and backyardgrown - Planning a wedding has enough stress, to try to grow even some of your own flowers would be a huge undertaking.
Most garden centers will have annuals available by April/May, you might consider doing some container arrangements instead of cut flowers.
Congratulations on the wedding - Good Luck!

Hi Sophia,
It is so exciting to grow your own flowers for your wedding. True, you do have many things to worry about around the time of your wedding, but at least you aren't relying on yourself to provide all of the flowers. I live in zone 7a and provided most of the flowers for a friend who got married 5/23/09. The following is what I grew for her: tons of Dianthus "Sweet William" (one of my favorite flowers), Larkspur (just starting to bloom at that time), a few Sweet Peas, Schizanthus "Butterfly Flower", a ton of Batchelor Buttons, a few of the following: Snapdragons, Baby Breath, Gaillardia "Blanket Flower", and California Poppy. As filler material: green seedpods from Honesty (AKA money plant), Hosta leaves, Spearmint, Ornamental Grasses "Miscanthus", Yarrow leaves, and Violet leaves.
It may not be too late to direct seed the Larkspur, Sweet Peas, CA Poppy, and Batchelor Buttons. Start indoors now: Dianthus. Although Dianthus is supposed to be a biennial, mine are still going strong after being planted 6 years ago. Also, look around your neighborhood and ask neighbors if they would mind if you took a few cuttings from their flowering shrubs, etc. I got married 06/13/09 and provided the flowers for my wedding but supplemented it with cuttings from a neighbors Hydrangea shrub, Lavender shrub, and gold Yarrow.Good Luck!

Carrie