Hello completelybonkers. What a lovely idea. Is it early or late October for your big day? Where do you live/garden?How much space do you have?
You could grow chrysanthemums to cut in October.
Woolmans have a good selection.
You might have to grow mid and late season ones, to give a chance of having some ready at the right time. It will depend on the weather between now and then.
Cosmos and rudbeckias should still be in flower then.
Have you a colour scheme in mind?
Dahlias will be in bloom - most are very bright colours, but there are also some lovely pale pastel ones.
Cosmos definitely - as long as you dead-head ruthlessly through the summer and don't let any of them get as far as setting seed!
I tried to grow my own gypsophila for my daughter's wedding last year, and it didn't succeed at all but you might have more success....
Let us know how you get on!