You can definitely buy
seedlings at a garden center. There is no need to start your own
seeds unless you want to save money or grow varieties that are not readily available as plants.
You could ask at a garden center when to plant for your area but they are notoriously wrong. Most don't know very much (except my mom!
). Try to go to a local news show website for local weather facts. At least our local news has a lot of info on last frost date and calendars and things. they also have a garden call in segment and stuff.
You can plant onions (usually as 'sets'- baby hard onions from the store) as soon as the ground isn't wet, sloppy and cold. The other veggies have to wait until the ground warms and the risk of frost is past. Herbs like chives are very hardy (member of the onion family) and can be planted soon. They stay green when it is very cold. Tender herbs like basil, cilantro, etc are very touchy to cold. Wait until you plant your tomatoes to plant those if desired.
My *guess* for your
zone is the onions will be ok now and the warmer veggies the first or second week of May to be safe.