You can get a 43 foot roll at Lowe’s for $1.33 here
I’m 100% sure you all know what dental floss looks like so I won’t even post a picture of it, you can probably get some at Walmart for maybe $2 or less. Cheap. All you do, is take one of the two, and wrap it where your o ring was. It’s trial and error. If it won’t fit, take off some. If it’s too lose, put more on. Simple. Do you have any suggestions for a makeshift o ring? Post in the comments!