if you are looking for tunes that fit your 20 button, try C major and G major of course, but E minor, A minor, and D dorian also tend to work. Those would also be the keys to aim for when transposing. Transposing isn't very hard really, if you take the time to write it out. If a tune is in D major, then transposing to C major is just a matter of moving everything down one whole step, so that D becomes C, E ->D, F# ->E, G-F, A->G, B->A, and that pesky C# becomes B. It is more challenging to transpose in your head while reading the original. It is possible, and I do it sometimes, but if I have time I often I write it out in the new key, or type the tune out in a program like MuseScore2 and then use the transpose function.
The Session.org is primarily (but not really exclusively) focused on Irish traditional music and D major is very common in that genre, so that is why you are seeing so many with a C#. You might try looking for English folk tunes instead, since G major tends to be more commonly used there in my experience. Also many American old-time tunes are in G major, since Banjo players often use an open G tuning, although Fiddle players seem to prefer A major. Scottish tunes tend toward A major, and Canadian tunes are often in A major as well, (Fiddle players again!) but G is common enough. You'll soon find that although these five genres have distinct styles, they also have significant cross-over, and there are many tunes that are claimed by more than one region, and some tunes claimed by all of them, although perhaps under different names.
Cajun music is another fun place to look for tunes that fit a 20 button. After all, the Cajun accordion is a one-row melodeon, which means that most Cajun tunes can be played using just one row of your instrument. The one-row Cajun accordion usually has a very simple base side, so the chords tend to be simple too, which is great for finding a workable harmony with the 20 button. For example, a Cajun tune in G can usually be harmonized with just G and D, or maybe notes from the G chord on the push, and notes from the D chord on the pull, and if the need for a C chord comes up on the pull, just slide by using that low G pull note. (note: Cajun music is related to Zydeco, but Zydeco music has more blues/jazz influences, so those tunes rarely fit the 20 button!)
Edited by Tradewinds Ted, 13 June 2017 - 10:54 PM.