I tried to post this earlier but it didn't appear...hope this works:
I am in the same boat you are, I LOVE 1-gallon batches; their easy, great for experimenting, and don't require much in terms of cost. I am also with you that 1-gallon carboys simply do not hold a full batch. You usually have very little space in the carboy and get an overflow during fermentation. DO NOT listen to people telling you to use cheap wine bottles. They can not handle fementation* A few solutions for you, since I have literally spent months trying to find a 1.5 myself:
1. Try different search terms and search engines such as 6 liter, 6 quart, and 6000 mililiters, these can lead you to better sites and not just to amazon, ebay and brew supply chains (which have none)
2. There is an antique carboy on ebay right now that looks like it is around a 1.5, very old and not a standard so check it out but be warned.
3. You need to head to scientific supply stores to get this done. I have found a 2.3 so far but they want you to order a case of 6 or 8 for over $600! I have found 5 liter glass carboys on there...
4. The * I put up there earlier. Buying some large cheap wine bottle is only going to get you a REALLY cheap pierce of glass. If you really want one that can survive fermentation, you need to get a champagne bottle. 1 1/2 gallon bottles are called an Imperial or a Methuselah (true champagne name). Of course, now that your buying one of the largest bottle of champagne on earth, you again getting into some pricey areas, but it will be exactly what you want. Go to your local wine store and see what's the cheapest they can order, get it and celebrate with a few friends on your spectacular ability to find a 1.5 gallon carboy when they really don't exist.
5. You can get one custom made by a packaging or container company, this might be expensive but I'm looking into it. They probably don't want to make only one, but there is a market for it so well see.
Hope this helps, send a pic if you get one!
Best,
Cali