Cant help you on where to find extract or flavoring, but have you thought about making a strawberry reduction?
Cook about a lb of strawberrys sliced in a cup of water and simmer for thirty minutes. Place in cheese cloth and extract all the goodness. Bring the liquid back to a simmer and simmer until you have a quarter cup left.
Ive never used it for wine, but you can add sugar and its great for desserts and sauces, strong strawberry flavor.
Again never used a reduction for wine, dont see why it wouldnt work though.
Edit for an alternative. Just occured you could make strawberry syrup by covering sliced berries with an equal part sugar and leaving it in the fridge for a couple days. Then fish the chunks from the syrup, give them a squeeze so you dont miss a drop. Might be good for backsweetening since it sounds you will be doing that.