Cadenzas on this site.

[rambux]

Am disappointed at the cadenzas included for the Mozart D minor concerto (no 20) and the Beethoven no 4.

Usually, the cadenzas included in the Mozart D minor are the ones by Beethoven. What do we get here? Brahms!

An even greater sin occurs with the Beethoven concerto no 4 - Beethoven wrote his own for this, yet again we get Brahms!

With the Beethoven no 4, I do at least have my old (crappy!) Schirmer edition with the cadenzas in it. But with the Mozart, I'm now reduced to having to compose one myself.

[stevebob]

There are many cadenzas in the public domain for the two concertos you mention.  The idea of writing your own is admirable, but falling back on one you consider crappy shouldn't be necessary; in both cases, there are numerous other options.