What kind of phone do you have is it a nextel device or a sprint device. Unless it is a Sprint wolrd phne it does not have a sim card but a memory card. You can get them at any store that sells micro sd cards like walmart If it is a SIM card for a nextel Phone I would advise goint thru telesales as Tom said All SIMS are not the same
Many Repair Centers have SIM cards you can buy for around the same price. Your best bet is to try an Indirect (third-party) service center. They are more likely to carry them. Some even carry used SIM cards for $5-10.
You can also find them on eBay for $0.89-$5.00