When did you receive the ticket? Have you responded yet?
If you already had a hearing and lost then you can try and call the court to ask if they can put you on a payment plan, or refer you to an agency such as Alliance One or Signal that can you can set up a plan with.
If you have not had a hearing, I would recommend requesting a hearing. The court will schedule a date for the hearing, and that will usually give you at least a month. You can use that time to pay the ticket, but I suggest that you actually attend the hearing.
You can request a hearing to mitigate the ticket, which means that you admit that you were not wearing your seatbelt, but you ask the judge to reduce the fine. Some judges will want a reason, such as a good driving record, economic hardship, or other circumstances that justify lowering the fine.
You can also contest the ticket - you say that you were wearing your seatbelt at the time. This may be difficult without a lawyer so you should consider consulting with one before you make your decision.
I agree with Mr. Jensen. In addition, you may be able to work off the fine through community service. A lot of the District and Municipal Courts in Washington State will allow that. Call the Court Clerk in the in the City/County where your ticket was heard and explain that you have a fine and that you would like to satisfy it with community service. It is worth a try. Good Luck!