^^^ this.
There are also two other approaches to think about.
One, if you can find polars for another similar boat (e.g., something near the 35-3's 13,000 lb displacement, 29ft waterline, 11ft beam, 600 sqft of working sail area, similar I/J/P/E rig dimensions, etc), they might be good enough to get you started. I know it is complete blasphemy to suggest that another boat might perform like an Ericson, but...
Two, you can build your own - not with a computer-model, but with real-world data. Keep a log of what works and what doesn't every time you sail. E.g. "11 knots true, close-hauled, jib-car is in hole number 3 on the inboard track, 6.1 knots at 33 degrees apparent wind angle". Do that enough times and you can build enough data to plot on a polar sheet.
Keep in mind that computer-generated polars are theoretical targets generated by a VPP (velocity-prediction program), and generally reflect a highly-optimized boat (perfect trim, perfect heel angle, boat not loaded with stuff, etc). They're nice if you're doing high-end racing - the polars will tell you target boat-speeds and optimal wind-angles - but... they're just targets. Knowing how to get the most out of *your* boat is worth its weight in gold, and the best way to do that is to pay attention to what it likes and what combinations of wind-angle/trim/etc make it sing.
$.02
