LOL - I thought you had access to the grape must, and that did strike me as odd in your neck of the woods (to say the least!). Grape must is generally available here because it is collected after the pressing of the grapes for wine and housewives and bread bakeries use it to make moustalevria and moustokouloura (a kind of biscuit made with the must). I do believe that petimezi can be used - it is basically a grape must 'syrup'. You may be able to find that at a Greek grocery, if you have one nearby.
I did find on the net that Italian Vin Cotto can be used instead of petimezi, so that may be more readily available to you.
I also found a recipe for you to make your own petimezi (provided you can afford to buy grapes this time of year).
Petimezi
INGREDIENTS:
5 pounds of seedless grapes
2 pound of granulated sugar (4 1/4 cups)
juice of 2 lemon
1 cup of water
vanilla extract
PREPARATION:
Day 1:
Put the grapes in a large stock pot and cover with the sugar. Sprinkle with the lemon juice, cover, and leave overnight.
Day 2:
Add the water, bring to a boil and simmer for 30 minutes to an hour or until the syrup thickens to the consistency of maple syrup (skim off any foam that rises to the top).
Stir in vanilla extract 1/8 teaspoon at a time to taste.
Spoon into jars when cooled and seal tightly. Store away from light and heat.
Note: A more traditional version of petimezi is made by straining the grapes through a sieve, discarding skins, and using wood ash as an ingredient; however, this recipe is is both easier on the cook, and just as delicious.