It's a bit late to be starting things from seed in your climate - particularly when you have a specific date on which you need blooms. Sunflowers (depending on the variety) probably take a good three months to get to blooming size and more time to actually set a flower. Mums you should be able to buy fully grown and blooming/about to bloom in all sizes and colors just about anywhere since they're a fall staple.

Best to have an alternate supply checked out - farmer's markets, roadside stands, fresh flower depts. at grocery stores, garden centers, Sam's Club, Costco, etc.

Here's a similar discussion. If you search "flowers for october wedding in maine you should get enough info to help the bride start planning.

For what it's worth - I looked at the link and noted the thread starter was beginning her planning in January... not nearly at the end of July.

Also something alse to take into consideration:
Zone 4
Average dates the last frost - 1 May to 30 May
Average dates the first frost - 1 September to 30 September

Zone 5
Average dates the last frost - 30 March to 30 April
Average dates the first frost - 30 September to 30 October

These are things both experienced and inexperienced gardeners have to keep in mind. So, my original suggestion still holds - which is simply to have a back up plan for a source of either cut flowers or potted plants.

I live in northern Massachusetts and am sorry to say that mid-October is not really a floral season here if you're planning a wedding. My best advice is to hit every Home Depot and other big box stores to buy mums and whatever else might be available IN BUD, not flowers, in September. It will be very hard to find mums etc anywhere that late--I have tried.

But the good news is that all the world travels to New England for our beautiful fall foliage in October. It's peak season here. Think leaves and berried branches and pumpkins and gourds and shocks of corn and a harvest theme. Farm stands are the place to go that time of year. The home magazines, like Country Life, have lots of ideas. Maybe you could go to the library and get some past issues for ideas.

Try to find an experienced local gardener to help you with suggestions. The wedding will be beautiful.