How do I buy the paper Ibond of $5k with my extension in addition to electronic bonds of $10K, since I am not filing my returns now and won't get a refund until I file.

I will need to first pay estimated taxes of $5k to create the refund situation (in addition to any taxes I may owe). But if I want to purchase the paper bonds now, how do I do it. The deadline is end of this month but I will not be able to file my returns before that.