Need window parts for Lockwood Double hung windows

I have have a house full of double hung windows. ( actually only the lower half raises and the upper half stays fixed) The spring has a plastic cover on the upper half which has become brittle and needs to be replaced. Lockwood originally from Canada has gone out of business as far as I can tell. Is that plastic sleeve at all a standard replacement part.for other window manufacture If so where would I find them.
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