Where can I get a free pair of guard rails for a bed?

I don't know about free, but you could watch Goodwill or other thrift stores. I ordered mine on Amazon.

For most seniors, the partial guard rails that slide beneath the mattress and have two metal legs to the floor are good. They can pull themselves up using the rail to a sitting position at the end of the rail. A nice thing about these rails is they don't cost much. You may be able to pick up something like this in an estate sale. If not, they are not expensive online as Grammyteacher mentioned.

If you're a member of a large church they often have things like this that were donated by members. Check with them to see. It's always nice not to spend money unless needed.

If you are asking for someone on medicare, a doctors prescription can get your LO a hospital bed that will come with guardrails if a hospital bed is appropriate for their care.
Call a medical supply house in your area and discuss the situation. If medicare covers, they will know and help you coordinate getting the prescription and delivery. Medicare will pay a monthly rental on the bed for several years and then the bed belongs to the insured. They may have separate rails as well, I'm not familiar with that but a large medical supply outfit will know all he ways they can provide products for a person with the issues your loved one has. If the first one doesn't, call another. They do this everyday so they learn the ins and outs and how to assist you.

Make sure you read up on the safety issues with guard rails. They can cause concern. Is the person who needs them still in Independent Living?