From the internet or locally. From the internet, do a search "91 Maxima Bumper Cover" and it should give you a list of car parts stores. as below:
http://cpwstore.carpartswholesale.com
http://info.automotix.net
Ask then if the bumper has been pre-painte in black. Pleas check if those sites are reliable. It must be a member of BBB and has a secure site for financial transactions (must take you to a secure site https:..., whenever it requires entry of credit card info). The shipping charge for a bumper could be high.
A better way is to go to a local body shop and inquire where you can buy bumper cover. If your bumper is still in good shape, you can ask them how much will it cost to repaint. Some shops will do it for @ $100. Don't limit yourself to one body shop.