You can go to any beauty supply store to buy double eyelid glue.
It comes with a fork for you to adjust the thickness of you double
eyelid. It can also be used as false lashes adhesive glue.
I bought mine when I was 16. I am 22 now and still using it. It
lasts me forever. lol..
I was one-side double eyelid and another side single eyelid. So,
i bought the glue and used for 3 months. The monolid side then
became double lid. The shape maintains till now.