My guess is they’ve simply outsourced the whole operation - design, configuration, logistics etc. - a perfectly sensible option given the ludicrously short turnaround time and the vast amount of work the rest of the club suddenly had to do.
A further guess is that the EFL or someone has arranged a supplier for this as an option for clubs to take the pressure off. Bolton are selling the exact same half and half scarf, which would back that up.
So I imagine we’re restricted by whatever that wholesaler is willing/able to provide. Though it would be interesting to see what they’d do if Norwich make the final.
I’m pretty certain you could get wholesale batches of yellow t-shirts in 24 hours though, so if there was the will and the capacity to do so, I’m sure it would have been achievable.