How can I generate voltage for a science project?
Buy power supplies.  No, really.  These things are produced in large quantities and sold into a competitive market.  Leave the safety design and regulatory approvals to someone else.  Anyone that has to ask here won't be able to design a "efficient and cost-effective" power supply compared to commercial offerings.  Even if they could, it would cost way more than just buying them unless the volumes are 100k or more.

Jameco is one place to look around since they have a very wide selection of power supplies, and they target hobbyists and other non-professionals.