Camco Stack Jacks, set of four 34$ and change, free shipping at Amazon Prime. Similar available Wally Mart, auto parts, farm/home stores. These are screw type jackstands; they are not jacks in the sense that they are not designed/intended to lift weight
although they will function in that manner if you apply enuf leverage to the crossbar.
There are also retractable folding stabilizers (about 16$) from Scamp
Trailers and other sources. Typically, two of these stabilizers are found welded or bolted to the rear bumper of small trailers, one each side. If not provided, a jackstand such as the Stack Jack is the appropriate substitute. You do need these when the tow vehicle is unhitched. 4 to 600lbs of human being in the rear dinette of a small trailer WILL cause the trailer to pivot on the wheels, possibly lifting the tongue at least momentarily. Not a pleasant sensation; even if the weight
isn't sufficient to upend the trailer onto the bumper, this movement can also dislodge chocks and cause wheel movement.
The usual tongue jack which provides leveling capability does not assist with lateral stability as you enter the trailer from the side door or move about. Customarily, two additional jackstands are placed near where the sides of the A-frame tongue (if you have one, some trailers have pole tongues) meet the parallel frame under the trailer.
The most frequent and primary role of jack stands is to stabilize the trailer in a single position and attitude and limit the effect of moving wgt. (humans) inside the trailer. I think the possible uses of jackstands (in conjunction with bottle jack for actual lifting or jacking) to provide safe and secure support during tire changes and for seasonal relaxation of torsion axles can be left for another time (or another ambitious contributor).