You can refresh the shop to search for your desired units, but it is expensive. Doing it too much in the early game will cripple you.
Fixed number of each hero available in a game: When someone in your game has a lot of some heroes, you are less likely to get those heroes in your shop. This means it is smart to take a quick look around the board and to try to upgrade units your opponents don’t have. When a player dies, his units get recycled back to the shop.