Just to let you know, the quantity of topping wine necessary to fill up your carboy to the correct level shouldn't be more than a few hundred milliliters provided you've got an assortment of different sized carboys and jugs to store your wine in. If you do the math, one liter accounts for about 4.3% of 23 liters. So let's say you use a full 750ml bottle to top up a 23L carboy, the wine you have added will now make up approximately 3.3% of the total volume. Provided you used a fault-free wine, the chance of being able to perceive any difference in aroma or taste from such a small amount is virtually nil. I understand the desire to keep your wine 100% homemade, but until you have a little stash of homemade wine built up that you can use for topping, I wouldn't be overly concerned about using a small amount of wine from the liquor store to keep your batch properly topped up.