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Good hardware stores carry spring assortments. Match by coil diameter, wire diameter and coil length. Test by checking the advance curve compared to the specifications either on the engine with a timing light or on a distributor test machine which probably isn't in most automotive shops anymore. McMaster-Carr is another source of springs, but they may want to sell in larger quantities than you need though their variety will be great. My owner's manual calls for 6 degrees advance at 450 rpm and 30 degrees +/- 2 degrees at 2000 RPM. My I&T manual shows fours differeent distributors and four different advance curves. My parts book shows three different distributors, different for Perkins, Continental in Deluxe and Orchard, and for the Special. But you said Z-145 so that makes it only Deluxe and Orchard. You may have to make the springs out of longer stock springs. Measuring with a micrometer or dial caliper may be necessary to get close and you may have to experiment with bits of the stock springs to get the desired advance curve. My I&T data for that Z-145 distributor is 0-2 degrees advance at 325 rpm (distributor shaft degrees and speed), 5-7 degrees at 700 RPM, 9-11 degrees at 1000 rpm. That translates to 0 to 4 crankshaft degrees at 650 crankshaft rpm, 10-14 degrees at 1400 crankshaft rpm and 18 to 22 degrees at 2000 crankshaft rpm. A little less than my owner's manual which is for a Z145 engine. Another source might be a distributor shop where they ought to have an assortment of springs and weights and the test equipment to pick them out. Might be disguised as a hotrod or marine shop. Gerald J.
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