You can get a pair of dead wedges for about $20 and change. Stops the bar rolling and useful for changing weight.
You can level the floor with self leveling underlayment (what I did, though one corner is no longer level after 2.5 years of deadlifting there).
Rip some long shims like this: Building A Lifting Platform on a Slope by Greg Everett - Equipment - Catalyst Athletics - Olympic Weightlifting