I have found that the sheet music sites for flute, trumpet, and sax work well. But also sites for Jazz with lead sheets, and there’s all kinds of stuff if you happen to have Band-in-a-Box (BIAB), from which you can print lead sheets with melody (and much more). It depends what genres of music you want to play. Basically, you want melody lines, preferably with chords indicated. The simplest version of BIAB is about $100, and it provides all kinds of accompaniment options; and there are sites with all kinds of BIAB tune files in all genres. BIAB takes an investment of learning it (gradually). I think the few books of sheet music specifically for melodica are pointless. Good luck, and welcome!