Seriously, buy a pass. The only way you'll find enough snow to build a jump is to go up into the MOUNTAINS, on a lucky day, in the deepest heart of the coldest darkest winter ever, you MAY find snow at Snoqualmie pass. Otherwise you'll have to hike a bit. No schools or parks will suit your needs. You must be new to the area, no?
We get snow in the lowlands maybe one day out of the year. Just buy a pass.