Hi allgeier44,
When it comes to proper GMAT preparation, there is NO substitute for regularly taking FULL-LENGTH CATs under realistic and test-like conditions, including taking the FULL CAT (with the Essay and IR sections), away from your home, at the same time of day as your Official GMAT, etc.
While it might be tempting to do a single section (to gauge your skills in that section), the score results would not be considered realistic, since you'd only have to face that one 75-minute section (and you wouldn't face any of the overall pacing and endurance issues that WILL be factors on Test Day). That all having been said, there are the
GMATClub Tests - they fit the parameters of what you're looking for.
GMAT assassins aren't born, they're made,
Rich