To culture isos just put them in a container, keep the substrate moist, and throw in some greens or slightly old food. Any mold that forms, they'll eat it! Bam! Isopod colony!
depends i use both in my hisser colones but in some of the other colones such as my glowspots or with some of my millipedes i just keep spring tail in there because thay will out compete the roaches and milli's
Also, you can buy both of them from Zephyr. I think the best clean-up crew is a mix of isopods and springtails. On Zephyr's page you can find more information about other clean-up species too.