Source for block-and-tackle window balances?

I'm interested in updating my existing 1908 wood windows, rather than replacing them. I'd like to get rid of the weight-and-pulley balances so that I can insulate the weight pockets. Of the alternatives, I like the block-and-tackle system housed in a jamb liner, like new windows or sash-replacement kits (e.g. Marvin Tilt-Pac) use. Is it possible to buy these block-and-tackle jamb liners separately?
Thanks, Wayne
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