For bulbs, potatoes and onion sets you can rely on the supermarkets to get their stocks in about a week before the right time to sow them so that is a good sign for those. I would also recommend that you go to a DIY place rather than the supermarkets as they tend to be half the price, as I found to my cost this year!
Organic seeds can be obtained from http://www.fermedesaintemarthe.com/ and they are the only providers here in France. Or http://www.organiccatalogue.com/ for UK suppliers.
http://www.growveg.com/Default.aspx is an excellent place to plan what you are going to grow as they have all the standard veggies etc and when you have chosen what you want to grow and where, it will give you a list of each plant and what to do (ie sow, plant out and harvest) for each plant. It even tells you when your frost dates are even though we are in France!
April is not too late to sow seeds, or you could go to the local market and pick up some small seedlings and plant them up. Lettuce, radishes, beetroot, carrots, peas, potatoes, onions, mangetout, tomatoes and courgettes (1 plant is sufficient of courgettes!!) are all easy to grow and can be started off directly outside. Herbs are probably easiest from seedlings from the local market and then put in pots right outside the kitchen door so that they are easy to reach when you need them.
Don't worry if something does not grow, sometimes that happens - I have always had failures growing leeks in the UK so I am hoping that I will be able to grow them for the first time here in France :-)
If you put plants out whilst there is a potential for frost, just make sure you cover them or bring them indoors on those nights, and if it is going to be really hot, make sure you water them either first thing in the morning or last thing at night. A little bit every day is better than a gallon in one go per week!