What about just building a dirt ramp or if no dirt available you can use the gravel that we call 0 to 3/4. which is gravel from dust up to 3/4 inch size. to make a ramp. I have used that for ramps on my garage bulding. Since I did not want it to spill off to the side too much, I built wooden retaining walls by staking some metal U channel down into the ground and fastening the wood 5/4 deck boards to that then placed the gravel to build up the ramp. It all sounds more complicated that it is to actually do, but if you have some dirt you can scoop up nearby with your front end loader, you can build a 2 foot ramp in about an hour. And if you want to tear it down, well just scoop it back up and remove it. I know that this is not what you asked about, some ready made ramps, but it is a cheap solution. You could also build your own ramps out of 4x4 and 2x12 stock if you wanted. But a dirt or gravel ramp is safe (when packed down and properly angled) and pretty cheap.