Before you request someone to model something for you, please try to do the following:
1. Try to finish your game or demo without it. Just put something else in its place and go on creating your game. This way you will....
2. Have something to show for your request so people don't feel like they are working for nothing. Not, "I need this for a game that I am going make", but "Here is the game I made and I need this to finish it".
3. Be reasonable. Don't ask for odd and complicated models and don't expect someone to work for free. That's why you should finish your game before asking, so you will have an idea of how much time and effort it takes to do these things.
I don't mean to put you off from asking for models or help, I only hope you will be considerate of others hard work as well.
I hope this helps.
Crazy Grandpa