Get the word out to your friends/family/coworkers that you are seeking gallon glass jugs. Other than that, what about cleaning/sanitizing something which will not interact with the ferment and "sink' it in the bucket? Simply displacing some volume. Do you have a glass pyrex bowl, or a few of them, some plates, etc. I know some who place bricks in double layer of ziploc bag to displace volume in bucket--I have never used that particular technique.
Or, ask at an area bakery, even at grocery store, if they can hold a few of their plastic buckets with lid (frosting, filling, etc) for you. They tend to be anywhere from 1-3 gallons, and free!!
Stop by the recycling center, if you have one, and see if they would allow you to take a few glass jugs?