If your lens resembles this:

http://www.epinions.com/Sigma_AF_28_200mm_f_3_5_5_6_DL_Aspherical_Hyperzoom_Macro_for_Canon_EOS__Camera_Lenses_787101

then it is likely not worth fixing, except as a hobby/learning process. Getting
parts will be interesting, usually one special orders the parts from the after
market service unit of the manufacturer. Many times this can only be done when
one has a special relationship with said unit (as in, approved service lab, a
status usually achieved after at least one tech takes a course with the
manufacturer).

The hardest part will be to ensure calibration at or near the ends (infinity and
closeup). Full testing of the lens may be required before you waste a film on
testing (and you can't see much in the viewfinder, you have to use a microscope
which is a part of the infinity jig). Do you have an infinity test jig ? 

good luck,
Peter



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