were can i find a gaming graphics card for low price for a Dell optiplex 740 desktop

You probably would not want to use modern GPUs anyway as they will probably bottlenck the old AMD CPU. If you have the desktop tower (the slim one) I recommend not even try to get a GPU as you can't really use anything good. For the mini tower an option could be an

ATI Radeon HD 5670:

www.ebay.com/.../222668622280

If you insist on a slim tower card:

www.ebay.com/.../122788455151

The ATI Radeon HD 2400 PRO will probably not give good performance but it will work on slim desktops.

I have a Dell optiplex 740 destop and i do alot of online  game such as  second life.But i need to find a gaming graphic card for my PC but  i don't want to pay a lot of money for an old rebuilt  Dell optiplex 740 windows 7 professional (64 bit).so any were i can find one that doesn't break my bank

If by desktop you mean this:

Then the best you could get is ATI Radeon HD 2400 Pro. I would not recommend upgrading it at all since this PC is very old and trying to put anything good will not work thanks to the 25 watt power limit. As the ATI I mentioned is no good.

If you have the Mini Tower though:

You could get somthing like a ATI Radeon HD 5670. I would not try to get anything newer as it will probably bottleneck the old AMD CPU,

ATI Radeon HD 2400 PRO:

https://www.ebay.com/itm/Dell-ATI-Radeon-HD-2400-Pro-256MB-PCI-E-Low-Profile-Video-Card-DVI-S-Video-YP477/132371061954?epid=129880823&hash=item1ed1ee1cc2:g:y7YAAOSwnTdZ67Ih ($6) 

ATI Radeon HD 5670:

https://www.ebay.com/itm/ATI-RADEON-HD5670-1GB-PCI-E-HDMI-DVI-VIDEO-CARD-Dell-HWHRN/222668622280?hash=item33d81589c8:g:pIQAAOSwbD9Z1qgu ($35) 

jennifer_1975,

When the Optiplex 740 was released Dell offered 2 different discrete video cards as an option. Below are the 2 video cards. 

ATI X1300 128 MB, x16 PCI-E

ATI™X1300 Pro 256 MBx16PCI-E graphics controller. This expansion video has a DMS-59 connection.

The system also shipped with a 280 W power supply. So, if you are looking at a larger video card you may need to upgrade the power supply as well. 

You can search the internet for one of these cards if interested in adding one of them.

The 740 uses AMD processor which is below Adequate for most current games. Older INTEL Netburst Pentium D Processors are not much better. The AMD processor in the 740 is not sufficient for most Current Games.

Asus EAH 6450 works with most all Dells.

https://www.amazon.com/Asus-PCI-Express-EAH6450-SILENT-1GD3/dp/B004X8EO6Q