I designed a junkyard brake upgrade for my RX8 using FK8 Civic Type R Calipers and 07-14 Mustang GT 355x32 rotors in the front and Mustang GT500 350x19 rotors with the stock caliper in the rear.
Most of the resources I used have already been mentioned, but to reiterate.
I used the Brembo catalog to narrow down the Rotors that would fit. The search tool is a little wonkey, but if you play around with it you'll be able to narrow down what off the shelf rotors will work best for your goal. The dimensional drawing are extremely helpful when designing in CAD.
Rock Auto's Catalog is also an extremely useful tool. While not every part has the key dimensions listed there's a lot of data centralized in one spot, but you need to have an idea of what make, model, year will have what you are looking for.
Centric tends to have good dimensions listed for their parts. Search their web catalog, usually the same data will be the rock auto listing, but sometimes there's a bit more on Centric's catalog
The GrabCad library has some useful stuff in it if you're making anything in CAD. I found a few models of calipers on there, as well as some rotor and hub models.
Google searches can dredge up useful forum posts with dimension, but be prepared to get creative in how you search. I spent hours digging through various sites and found some good nuggets of information, but be careful as occasionally the information posted is plain wrong or may send you down the wrong path.
If you strike out on all of the above you can get measurements off images. It's not going to be exact, and requires a bit of effort and luck, but it might help narrow things down.
Look for images of the part in question. Try to find an image that has the part oriented so that the dimension you want to measure is centered and as flat as possible to the plane of the camera. There will need to be a reference point/object that you know the dimension of in the frame. A bolt head can work or another dimension of the part you already know; references with sharp defined edges are the best. Try to use medium size reference objects as measuring error will tend to increase with small objects. It'll need to be close to the same plane as what you are trying to measure.
The manual way of doing it is to print a copy of the image and measure the reference point/object in the image with a ruler or caliper and divide your known reference value by the value you measured on the image. This will give you the scale ratio of the image. Measure the dimension you want to know and multiply it by the scale ratio to get the actual dimension.
There is software you can use to do it digitally. I've used Adobe Acrobat to do it before, but there are many free open source programs available better suited for it. If you have Fusion360 you can import the image as a canvas, scale it and get the measurements and/or use it as a template for a sketch.
I measured out the RX8 knuckle, hub and brakes and used that as a basis to build my adapter brackets in Fusion360. I built models of a few different rotors based on the drawings from Brembo's catalog, and imported a couple potential caliper models to see how they fit my design.
Once I selected the caliper and rotor I was going to use I 3D printed the adapter brackets for mock-up to make sure I had the clearances I needed on the car. The final design was cut out of steel by SendCutSend and it bolted on with no issues.
