It was the 1996 to 2000 time frame, that Dell was proprietary with the 20-pin ATX main power cable, and the matching connector on the mobo. (MOtherBOard).

Standard 20-pin ATX main power cable, and it's corresponding connector should work, then add the 4 power wires that are missing, in a separate connector.

To expound:

View this standard 20-pin ATX main power cable, and it's respective connector,

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain20

The power wires should be in the same place as your P3 units.

(Match the color codes.
Use the lock tab on the 20-pin ATX main power cable connector, to base where the pins are.

Pins 1 though 20. Match the color code of the wires to the pins, and use the chart to compare)

Here is a view of a standard 24-pin ATX main power cable, and it's respective connector, so you can see what power wires are missing, from the 20-pin ATX main power cable connector, that are present in the 24-pin ATX main power cable connector.

http://www.playtool.com/pages/psuconnectors/connectors.html#atxmain24

Note the missing power wires, and what pins they line up with.

The P4, or also stated as the 4-pin ATX +12 Volt power cable, uses two Yellow (12 Volt) wires, and two Black wires, (Ground)

http://www.playtool.com/pages/psuconnectors/connectors.html#atx12v4

Since you don't have the male connector, be sure to orient in your mind where the Yellow (12 Volt) wires go, and the Black (Ground) wires go, by looking at the lock tab on the side of the female connector, on the motherboard, and comparing it to the photo.

Transfer this info, to the P4 connector you are going to make.

Where to get the power wires you need? Connect to any extra 4-pin Peripheral power cable wires, or splice off of ones that are in use, is one suggestion.

http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral

The links given are near the middle of the page. Scroll back to the top. Left-click on any power cable connector, that you wish more information on.

More info,

http://en.wikipedia.org/wiki/Computer_power_supply

Scroll to the bottom of the page, and look at the chart under the heading -

24-pin ATX12V 2.x power supply connector
(20-pin omits the last four: 11, 12, 23, and 24)

One more link,

http://www.smpspowersupply.com/connectors-pinouts.html

Lastly Dell Support > Optiplex GX260 > Service Manual,

http://support.dell.com/support/edocs/systems/opgx260/en/sm/index.htm

Just in case it's of any use.

Have questions regarding what I have stated, or need more clarification of anything I have stated, simply post in a Comment.