Nismo Hard rubber motor mounts - http://www.frsport.com

JGS rubber mounts - http://www.jgsturbo.com

Megan racing rubber mounts - http://www.enjukuracing.com

Stock motor mounts - http://www.courtesyparts.com

Window welded stock motor mounts - http://www.homedepot.com

Polyurethane is much stiffer than the stock rubber. You'll need to stab a hole in the mounts and inject them with window weld and allow them to dry/set. I'd personally go with the JGS mounts. Best bang for your buck if you're looking for something mildly stiffer than stock.