Where can I find just part of a bedding set?

I have registered for the Sumersault "On the Go" bedding at Target. However, we do not want to the 4 piece set since we only want 2 of the 4 items. I can register for the sheet separately, but so far I haven't been able to find the dust ruffle on it's own. Does anyone have ideas where I can look or where they were able to find this piece on its own?

Thanks!