What kind of vehicle? Are you lowered? What are you trying to accomplish?
Are you trying to get under the car just with ramps, or are you wanting to use the ramps in conjunction with a floor jack? The plastic ones from Wally World/Autzone aren't really sufficient for a car with low ride height, not even the "low profile" ones. Plus they tend to slide.
My car I made my own ramps to get the front end high enough to use my floor jack under a central point.
If you can't fit a jack under your car and don't feel like making your own ramps, something like this would be ideal:
http://www.z1auto.com/prodmore.asp?model=rx7&cat=handling&prodid=1304[This message has been edited by madd_ag_05 (edited 1/20/2013 9:09p).]