I've not seen anything like that for Corel. You might be able to find DWG files for some things but it's very easy to build your own library. PNGs, BMPs and JPGs will be useless since you don't know if they have been re-sized and they won't raster
No need for trial and error, get yourself a cheap caliper, dial or digital, and measure things or see if the dimensions are listed in the specs.
Here's how I'd do a linear pot.
Measure all the bits, IE, hole diameter, distance from hole to hole (measure from bottom of one to bottom of other), travel of slider and width of slider.
Draw a circle, click the circle tool, hold the Ctrl key down and drag a random size circle. Making sure the lock is on in the object sizing area, enter the diameter of the screw in either box. The lock means it will keep it's current aspect.
Set the duplicate distance equal to the hole spacing, Ctrl-D and you have a second one, at the correct spacing.
Select both circles and Ctrl-G to group them.
Draw a random sized rectangle, unlock the object size controls and enter the correct dimensions.
Double click the arrow tool to select all and press c then e. Now everything is centered in both directions.
Ctrl-G again to group all three objects and save as linear pot size xyz.
70W Epilog Legend 24, bought it new in December 2002