I just filed online but will pay by check. How do I get a payment coupon to send with my check, or do I just send my check?

It should have printed out a payment coupon to send with your check.   If you printed your tax return out it should be there.  If you didn't print your tax return out, reopen your completed file and go to print and review your tax return, it should print it then.

AI M1 is correct. TurboTax should have printed the 1040-V voucher. 

I recommend you pay your tax payment directly, using Direct Pay, on the IRS website. You'll be able to confirm immediately that the IRS has your payment and you won't have to worry if your payment is "lost in the mail".