I always get some from the local shop(s) or grow from seeds I harvested the previous year. My garden is weird - 2 years ago I was overflowing with huge Charleston Grays, but last year it was the Sugar Babies that dominated and persisted. Sugar Babies are small and probably have the same amount of seeds as a big melon (so proportionally more seeds), but they're super sweet as the name implies, and that gives them an extremely forgiving harvest range. I.e. if you get them a little early or if it frosts before they're fully ripe, they'll still have a good flavor.

Anyway just research whatever variety you're buying. I find all seeds are pretty cheap for the yield (like $1.29 for some 50 seeds which yield at least as many melons if you have the room). Holy shit I just checked Amazon and it's like $7/100 seeds... you're overpaying for sure. Still I mean, a small investment for how easy melons are to grow.