Where can I find a library of relays automotive flat pin with pin 6mm x 1mm?
Normally you would use a socket with a relay like this allowing round through holes in the PCB.
Here is a VCF4-1000 socket:
You can draw composite pads with slots in Eagle, but may end up with DRC overlap errors that need to be waived.
Here is what the manual suggests:
8.13 Components with Oblong Holes
If the board manufacturer has to mill oblong holes, you have to draw the milling contour of oblong holes in a separate layer. Usually this is layer 46, Milling.
The milling contour for components that need oblong holes can be drawn with WIRE (and possibly ARC) with a very fine wire width near or even 0 in the Package Editor. Take a pad that has a drill diameter which lies inside the milling contour, or SMDs, for example in Top and Bottom layer, as basis for the oblong hole.
In case of a multilayer board you should draw a WIRE in the used inner layers at the position of the oblong holes so that it covers the milling contour and leaves a kind of rest ring around the opening.
Please inform your board manufacturer that they have to take care on the milling data drawn in this layer. Also tell them whether they should be plated through or not.
Any other cutouts in the board are drawn in the same way:
Use a separate layer, typically layer 46, Milling, and draw the milling contours. Tell your board manufacturer that they have to take care with this information and make special note.