I'm in no ways a professional artist, but the way I try to find nice looking UI elements is search istockphoto, and sometimes google images for vector UI - and re-create similar ones using Illustrator/ inkscape.
Not super custom, but it looks nicer than an otherwise flat gray box 😉
Also if it's your first mobile game- realize you might have to make several before you touch on a good niche market.
My longest game was an angry birds game- in space, started on like 6 months before angry birds space was announced..... I released it early, with some basic levels finished, but didn't get much success with it. A similar game, 'iBlast Moki 2' was featured on the google play market homescreen for about a month- and they got dismal downloads- something like 10k total downloads.
It, just like our game, had more fun gameplay than Angry birds, but for some reason the market at that point, for that game was over saturated, and got really terrible download numbers.
So hopefully your game does well, and it earns enough for you to keep working on it + creating new games.
But it's always depressing to see a "toilet paper dispenser app" earning enough to let the creator create 100 more apps like that, and cool games going unnoticed.