First of all, thanks for the kind words and I'm glad you're finding this forum useful -- we greatly appreciate the constructive feedback!
Based on some work by extension specialists at University of Arkansas, it appears they found Dow's Grazon P+D herbicide (Picloram + 2,4-D) to be very effective in treating honeylocust sprouts -- they used a 1% foliar solution, applied to both honeylocust and black locust seedlings.
Regarding basal applications, Triclopyr (ester formulation mixed with basal oil) is your best bet. Garlon 4, Garlon 4 Ultra, and Pathfinder II are two examples of triclopyr-ester products from Dow AgroSciences. Garlon 4, at 25-30%, mixed with basal oil and a colorant will do the trick. Pathfinder II, on the other hand, is a Ready to Use (RTU) version of Garlon 4 -- it comes pre-mixed as herbicide+basal oil, but Pathfinder II typically does not contain a colorant.
Some "forestry" basal oils contain an oil-soluble colorant; however, this varies by manufacturer. Basal oils and colorants are relatively inexpensive when compared to the cost of the herbicide. Examples of basal oil/colorants include Townsend's Ax-It & Trail Lite 2000.
Yes, you can apply Tricloypr-ester herbicides during the dormant season as basal bark treatments -- the oil-based herbicide and oil-based carrier -- penetrate the bark of trees < 6" in diameter.
Where to buy in Illinois? Local FS dealers; Rural King; and Farm & Fleet. Your FS dealers will likely have a much larger selection of forestry-, pasture-, and CRP-labeled herbicides. Don't be afraid to try some of the generics out there.
Hope this helps!