Coupons.com, Red Plum, or Smart Source.
If you decide to print coupons at coupons.com, make sure you are printing them from your Inbox Dollars account. When you do this, you can earn $0.10 back every time you print and redeem a coupon through your Inbox Dollars account. It's free to sign up for an account.
If you are specifically heading to Target, be sure to print coupons from Target here.
To save money on printer ink when printing coupons, I suggest setting your printer settings to grayscale or black and white, using "quick print," and using refill ink. Stock up on printer paper when it goes on sale.