Tissue, cell and organ donation (including blood, semen and oocytes) can and should be done strictly not-for-profit. This is how it’s done in Spain (well, you do get a snack when donating blood and a small amount of money for oocytes since the process is quite long) and there’s usually no shortage of blood components in hospitals. Local governments do a lot of campaigning, set up mobile units etc., which seems to work; people see all of that, think of it when planning their day, and many even go in small groups to donate.