I'll need to explain further before I can recommend a location and you'll see why.
2-4-D, one of the most effective broad leaf targeted weed killers has been moved to "Class B" status in a ton of states for products that contain more than 4% of the active ingredient. For reference Class C is for homeowner use, Class B requires a professional to apply and Class A is restricted. That means 2-4-D (4 to 5% base solution) weed killers can still be applied and purchased only by professionals who hold the necessary certification.
The problem you will encounter is that many big box stores have voluntarily stopped stocking herbicides, even in states which allow them. Take Home Depot for example; you'll go into a store and all you'll likely find is Round-Up, the ecologically safe crap and a few weed and feeds. This is mostly due to fractured distribution channels since so many states have banned targeted herbicides.
Luckily, Wal Mart has no such compunction. You will likely still find Spectracide in their stores in Vermont (Burlington).
For example:
http://www.walmart.com/ip/Spectracide-W ... y/35789535
http://www.walmart.com/ip/Spectracide-W ... e/35789534
http://www.walmart.com/ip/Ortho-Weed-B- ... l/34972932
http://www.walmart.com/ip/FASTTRACK-Ort ... ./20470657
If you have grubs, some municipalities still allow the strong stuff. My area in Vaudreuil / St Lazare allows garden centers to stock Grubex and Merit, two of the most powerful products on the market. You can find those at Les Serres Vaudreuil, Pepiniere Cite des Jeunes, etc.
Hope that helps.