These guys are right "hoochie momma" is by far your best bet they sound real and are basically fool proof.
If you have access to somone who can teach you then the next step and a good one if you can learn it properly is a call similar to the Sceery Hyper Hot cow call. Most of the call companies have one like it. Your choice would probably work. Easy to blow very difficult to not blow it. If you do not know what you are doing you would be much better off with the hoochie momma.