There is no way to get SkyTest for free unfortunately, have you tried LatestPilotJobs, or even better -
PAT | PilotAptitudeTest.com - Practise for COMPASS Tests and PILAPT Tests?
Both are cheaper alternatives to SkyTest, and I'd say PilotAptitudeTest.com blows both of the others clean out of the water. Failing that, check your local pilot supply shop, there are books specifically written on passing CX assessments.