They way the weather has been yo-yo-ing around here, I dont think its too late. I'm on border Qns/Nas and I'm still putting things in.

Most locally native wildflowers need a cold period to soften their seed shell. (Some can instead be heat-treated, but this varies quite a bit by species). Planting these may push you into June, which may still work, but I'm not sure.

However, most wildflower mixes you buy at the store have non-native annuals to add first-year color. These will probably work from seed, but they also usually have some very weedy species that might take over any wild lands nearby, hurting the environment and making them very difficult to eradicate if you need to later. It's always a good idea to check out whether the mix has any invasive species. Otherwise the headache of removing them later may be worse than any initial weeding you are doing now (though of course that is your choice based on your situation).

Just to let you know, the Long Island Native Plant Initiative is having a plant sale June 3rd, 4th, 10th and 11th. They are the only seller of Long Island local ecotypes that I have ever found. The plants come in flats, and still may not bloom this year, but if you want to speed the process of making a wildflower garden along, this is one way to go. Here's a link: http://www.linpi.org/