I have 8 children, and I have used many birth control and did not work. I have vascular veins in my legs and they may get worse if I become pregnant again. I want to ask if it is permissible to get an operation called "tubal sterilization"?

Praise be to Allah, the Lord of the World; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
If pregnancy harms you greatly and you can hardly bear it and if you fear for your own life and if you have gone through all available ways of temporary birth control but in vain, and you no longer have any solution but to undergo a tubal sterilization operation, then it becomes lawful for you to have this operation as to save your own life and to avoid being confronted with great hardship. Allah Says (interpretation of meaning): {…And do not kill yourselves (nor kill one another). Surely, Allâh is Most Merciful to you.} [4:29] and He Says :{… Allâh intends for you ease, and He does not want to make things difficult for you. …}[2:185].
Allah knows best.