Need replacement one-piece window roller

A quick learner from San Francisco, CA says:
Hello, I have sliding windows in my room. I realized that they are not closing properly because the rollers on the bottom are worn and causing it to sit unevenly in the frame. The window itself seems to sit on top of this one piece roller that is not attached to the window, it just fits neatly on top of the piece here, held by the raised bits on the ends. The piece is 29 3/4" long and 5/8" wide.
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Paul from SWISCO responded:

Many thanks for writing in. It is our belief that you can just replace just the ends with either the 87-057 or 87-213 . Please compare and let us know your thoughts.

A quick learner from Denver says:
Can you buy new roller channels complete?
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Bob from SWISCO responded:

We carry a very limited selection of sliding window guides and tracks here . This hardware is very difficult to stock replacements for, since it has to be very precise in design and many options are no longer made. A local hardware store familiar with sliding windows used in the San Francisco area may be your best bet. I apologize we cannot be of more help. 

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