Unlike the 1911 or the Glock, the Desert Eagle was NOT designed to be a military firearm. It was not designed to be constantly taken a part and put back together. And while its not even in the same category as a finely fitted DA revolver, its not a "throw parts in a can, shake well, and remove pistol" class, either.
As others have pointed out, there is no market of purpose built aftermarket parts. There is only ONE maker. Parts at Numrich are from guns people have sold them for parts. Their diagrams will show you all the parts, but, particularly with certain guns, they don't have all the parts. Also, they don't sell frames (generally) and if they do have a frame for sale, it has to be sold as if it were a complete gun, with all laws applying.
you could probably locate a functional frame from somebody who has irreparably soiled his gun with cast lead bullets (apparently the gas system cannot be cleaned).
You don't "irreparably soil" the entire gun from using cast bullets, but you can PERMANENTLY plug the gas system. A new barrel assembly with return the DE to operating condition. The downside to that is it costs close to 1/3 the price of the entire gun (or it did 20 years ago
).
Unlike other gas operated guns, the DE ports gas from just ahead of the chamber, and the tube it travels through is a tunnel under the bore, Not a separate piece that can be removed and physically accessed for cleaning. Nothing at all like an AR. If you clog the gas tube in a DE, ALL you can do is use liquid solvents, and hope they do the job. Sometimes, they do. Sometimes, they don't.
One DE I know of had to go back to the maker to unplug it, and when it came back, it came back with a new barrel, and a bill for it. Its NOT a warranty covered problem.
IF you are able to get all the parts (even over time), AND you have the skills to do the fitting that will be needed, the odds are still very high that in the end, it will cost significantly more than buying the gun used, and very likely to exceed the cost of the gun new.
If you want to play Dr Frankenstein, choose a 1911 pattern gun. ALL the parts are widely available (you won't have to rob graves), and the huge market means the parts will be as cheap as practical. Also there is a wealth of information on how and what to do, and not do. The 1911 is one of those guns where there is little if any fitting needed for a functional assembly. Some might be needed, but if the stars line up, none will be, and the gun will be functional, and safe (assuming you don't get crap (out of spec) aftermarket parts).
And yes, with the DE, the magazine MUST "float" or it will malfunction. They are a very specialized piece, and one has to treat and shoot them they way they want, sometimes quite different from a regular service autopistol.
If you are going to go ahead with a DE build, GOOD LUCK!