The way you can be really sure you have neonic-free plants is to buy organic seed and raise the plants yourself. There are several good sources of organic seed online including Seeds for Change and The Natural Gardening Company. I am sure there are many others if you search online. Not only do I grow all my own vegetables from seed – some warm-season plants like tomatoes and eggplant under lights in my basement – but also some annuals like zinnias and snapdragons.
In addition, there’s a wonderful native plant nursery in Minnesota that sells online and will ship plants to you. It is Prairie Moon Nursery. They do not use neonicotinoids. The wholesale nurseries that supply the CFC's annual plant sale also do not use neonics.
I also understand – and you could call to check – that Pesche’s in Des Plaines has all organic plants. I know they are a very environmentally friendly company and even use rain water to water their plants.
Beyond that, you can ask at various nurseries, but they probably won’t know or won’t say. They may not know how their suppliers grow the plants they provide.
Thank you for your question. I hope you have good luck with this. Let me know if you run into anything interesting during your search for pesticide-free plants.