Was going to ask you what you were using your car for, and what condition the brakes are in as standard c brakes in good condition aren't bad.
Having seen your profile pic I understand and guess your getting quite bad fade?
Theres 2 ways of doing jobs like this, a cheap way and an expensive way, the cheap way is to go wandering scrap yards looking at different disks and finding one with hopefully the same pcd (although this is less important, the holes can be re-drilled) but most importantly the right bell depth and diameter to fit your calipers, eg. Peugeot 505 turbo front disks are vented and fit on the front of an MGB.
The expensive way is to see if anyone makes a kit, which on the C I doubt, unless any yanks have done it.
Rear disks may be easier as the rear drums are capri, I'm certain someone will have made a capri rear disk assembly, which with luck should bolt up to a C back axle with minimal work.
Hoyle would be worth a word with, as they do these conversions for MGBs, and some parts may cross over with minimal work. Also maybe Pete Burgess.
Don't forget it isn't simply a case of buying or making a kit and bolting it on, you need to make sure front to rear bias is correct for the car, and that the fluid displacement from master cylinder is sufficient. As well as making sure your wheels will fit over the calipers etc.
Unless you have already done it for now I would be looking at finding ways to increase cooling and increase efficiency of the existing brakes first.
Yes the fade is pretty bad, although, we fitted grooved and drilled front discs for Thursdays session, and there was a bit of improvement, although not vast, so not all doom and gloom.
As the C is quite nose heavy, the rear drums aren't all that bad, setting up the bias will be a pain though, but I think having rear discs would definately offer an improvement. I will look into the Capri situation.