How to order TRD mudflaps?

I don't think they are on sale yet. Took a look at the TRD site and all I could find were oil caps, filters, and air box top. I'm guessing they've released the renderings of their kits only and haven't actually put them up for sale yet.

Here's all they have: Toyota Racing Development

If they are a TRD product, it'll be a dealership order that they may have in stock in Japan. Best bet is to call the parts department and ask them if they can give you an estimated time on when they will be getting them or if you can order them in.

In regards to gas mileage, I haven't heard of anyone noticing a difference at all due to mudflaps.

In theory there would be a slight increase in wind drag, but you won't notice it and your own body weight is more likely to affect the C-HR's fuel economy rating than four mud flaps.