Summarize:

Typically, only mail carriers along rural routes sell stamps, but you might as well ask your mail carrier. Rural route carriers usually carry enough stamps to meet their customers' mailing needs. Avoid simply leaving your mail and change to cover postage in your mailbox. If your mail carrier doesn't have stamps for sale, you may be creating extra work for him/her. Your mail carrier will give you the form to fill out or pick one up at the post office. Once you've filled out the form (PS Form 3227-A) with the type and number of stamps you'd like, include a personal check or postal money order. There is no charge for stamps by mail. Use the Business Reply envelope to avoid using a stamp to order more stamps. Make the money order out to "United States Postal Service". Never send cash. Your mail carrier will deliver your stamps in a few days.
Ask your mail carrier if he/she has stamps for sale. Ask for Stamps By Mail form. Mail the form and payment to the post office. Receive your stamps.