How do I let an employer know about my condition without ity hurting my chance to get hired?

I am on SSI and SSD due to chronic back and neck pain wich doesn't allow me to stand or sit longer than two hours. I have many people tell me that when I apply for a job, not to mention my problem in order to get hired, however, if they don't know that I cannot stand or sit longer than two hours, how would I ever do the job if they hired me and expected me to stand throughout an entire shift? Without them knowing, I would never be able to do it. I have mentioned my condition to employers in the past, however, they refused to provide me with a stool so I wouldn't have to stand, and they passed on hiring me for the position that I would otherwise be able to do. How do I get around this while still being honest?