Fertilizer is typically needed to add nutrients to soil that's lacking and adjust the pH level to the optimum level for the grass being grown. For lawns, the best pH level is between 6.0 and 7.0, but the exact level depends on the type of grass being grown. Not all grasses are alike and some thrive on certain soil conditions that others won't tolerate.
Test the pH level of your lawn to see what kind of fertilizer you need. If it falls with in the acceptable range of 6.0 to 7.0, then you more than likely don't need any fertilizer. Check the information that comes with your particular seed to make sure. The acceptable range is just barely acidic. If the number is lower that 6.0, you will need to add a base to lower the acid level. If the number is higher than 7.0, you will need to add acid.
Read the grass seed instructions to see whether the seed requires any particular nutrients. Choose your fertilizer based on the pH level and what the seed needs.
Look for fertilizer at home improvement stores, garden centers, super stores and nurseries. Use a phone book to look them up and find addresses if you don't know where they are located. Or you could search the Internet to find locations. Call the store to make sure it has what you need in stock before driving there.
Order fertilizer from an online retailer and have it shipped to your address.
Make your own fertilizer with a mixture of epsom salts and ammonia. Mix one part epsom salts to one part ammonia to create a concentrated mix. Add one tbsp. of the mix for every gallon of water. Use a sprayer to apply it at a rate of 1 gallon for every 100 square feet.