Is there any marketplace in the internet where I can find/request custom sprites/assets required for game development?
http://opengameart.org/ will have what you need to get started.
Some 2D and 3D websites have a request forum, however there is no obligation for any of the forum users to make what you want.
You can form a team with a willing artist, although any obligation they have to make art for the game will reflect your own obligation in developing the game. here on Gamedev is a Hobby Projects section you can try, or just ask around on the net.
The best option would be to hire an artist, this way you will get what you want as long as you can pay for it. Finding professional artist is a bit harder as they don't often check forums for work, there is portfolio sites that you can check to find people and you could use the Jobs section in most forums to find artist.