They are called low profile cards, the choice is very limited. Heres a very good one click here but it may be more than you are willing to spend, heres another click here and some others click here
What you need is a PCI Express low profile graphics card.
One warning. Any low profile card should fir and run ok but the more powerful cards might need a better power supply than you have so I would suggest going for one of the cheaper less powerful ones.
Can I ask what you want it for?