Until you find the jars a plastic ziploc bag will work. Gallon size works pretty good.
I usually find the half gallon jars in grocery stores or places like Walmart, Target or such. Usually in the canning isle, but sometimes way in the front of the store in front of the registers. Most of the year they sell the small jars, but during the summer and fall they have the larger ones too, because it is canning season. If you don't find them just ask at customer service and someone will show you where they are.
The gallon jars I find at yard sales. I've not seen them in any store, at least not empty. They are usually used for pickles now, but used to be used for mayo, too. Most large package stores, like Smart and Final or Sams carry them with food included. After the food is used up just run them through the dishwasher and you are ready to go. Make sure the food in them was GF.
I think Elana has a listing on where to get the jars. I'll have to go look and see. I just looked and this question is under FAQ, products.
HTH