Officially, the only answer we're allowed to give on here is to buy an install disk-Ebay is probably the easiest way to do this.
Unofficially, there reputable places on the internet where it can be found. One might look to a garden of Macintoshes.
As a side note, the standard Cube configuration had a DVD-ROM drive, but a CD-RW was optional(I seem to usually see them on 500mhz models, but they could be had on both). The standard distribution of 10.4 was on a DVD, but since there were computers that both supported Tiger and had a CD-ROM drive. Consequently, you could exchange the DVD for a 4-CD set at the Apple store(I have some original CD sets, and it looks like they were burned in the store on demand). In any case, images of both versions can be found on the internet-find out what type of drive you have, download the appropriate one, and burn it.
It's actually also possible to install off USB, but this will be a slow process over the USB 1.1 connection in the Cube.