How to get Leopard installer on USB drive?

I think as a rule the G series Macs were not USB bootable. There's a couple of posts on the Internet about people being able to do it with some models but generally no. Being able to boot with USB was really a features of Intel Macs. You booted G5s to optical drives for installing or to an external Firewire drive.

Apple doesn't even have Snow Leopard anymore. You will have to look on web sites for used computer equipment. If you do buy some, make sure they are black colored retail installer discs. Machine-specific discs (gray color) may not work unless you are lucky and get the right flavor.

It would also be helpful in this instance to know which exact Mac model and year is involved. I don't know about Leopard but sometimes a retail installer disc won't work if a Mac was provided with that major version. For example, retail Snow Leopard was 10.6.3 but some Macs supplied with Snow Leopard took a later 10.6 version and wouldn't boot to retail.

Have a look at https://everymac.com/

to discover the needs of your Mac.

Welcome to the Apple Support Communities.

You need an OS X Leopard install disk. Do you have one?

Cheers,

Jack

Find a Group --> http://appleusergroupresources.com/find-a-group/

Mac User Groups are local Apple clubs and can be a handy resource for getting old equipment up and running.