Welcome to WMT. In terms of your question, I have two thoughts. First, if you're talking about fortified wine made with brandy, the cheapest way to buy it is probably at your local store. For example, you can use a regular Gallo brandy made from grape wine, adding it to a red wine.
Beyond that, though, I haven't seen much here on fortifying, probably because of the cost involved in that process. It would take about 8 bottles of 80 proof (40% alcohol) brandy (750ml each) to raise 5 gallons of wine from 10% alcohol to 20%. That would probably cost over $100 for the brandy, raising the cost of each bottle of wine significantly.
BTW, other alternatives include blackberry, peach, or blueberry brandy added to a similar wine.