don't know your climate, whether Dahlias like it. July could be somewhat too early,
I plant mine mid- late May, in July they just about start flowering (need to check my fotografs)

apart from that: you would`t bother with seedling Dahlias, as they are of mixed parentage and will show all kind of different colors.
and it could ruin your timing, assuming that you don't have a greenhouse at your disposal?

The Dahlia seeds sold in garden centers are often from short ones (perhaps additional filler flowers) but nothing really amazing.

Do you know what kind of colors you need/ want?

I would decide on the colors, then pick some named cultivars.
I gather that growers like Swan Island (did I get that right?) have recommendations for cut flower cultivars (long stems, long vase life).

quantity is tricky,
mine (mostly water lily types) produced about 2-5 flowers when harvesting about once a week.

you could ask on the cut flower forum about quantities, lots of people selling theirs on the market so they could give you a better guess/ calculation.

did you grow Dahlias before?

Well, best of luck,

bye, Lin

I am in zone 5/6 Pennsylvania, and the earliest I've gotten dahlias that we're suitable for cutting was July 15... But that was also starting them from tubers mid-march so they were 12-18 inches tall before setting them out. Even so, most of mine were blooming mid August.

Remember, if you are thinking of purchasing tubers, that typically the larger the flower, the later the bloom.

The vendor Lin mentioned also sends dahlias all over the US at decent rates. If you choose to try growing your own dahlias,that is great! You'll have lots of fun learning about them! BUT have a backup plan in case that cute bunny over in the neighbor's yard comes over for a dahlia snack before the plants can fend for themselves!

Best of luck!
CC