As far as i have been informed its legal to sell amps. You can buy them online, i have seen them in CB section in truck stops, a CB radio shop here in socal sells them. If they are still illegal the feds are not inforcing the law. In the 70's many CB radio guys in south mississippi had tall antennas in their yard with tube amps connected to their CB. I never knew of anyone to get busted.
It is probably technically legal to sell amps for any frequency range, just illegal to use them. Some people like to claim it is actually for ham radio to keep out of trouble since they are similar frequencies.
Most legitimate ham radio operators are probably using 5W-100W radios in their vehicles.
15KW no way ! First off that antenna would be doing good to handle 5000 watts. A Texas Star "Hot Plate" is 1000 watts and about 12" L x 7" W x 5" H and has 2 fans. A "Hot Plate" needs at least a 130amp alternator to operate without dimming factory old fashion low beam headlights. How do I know ? South Mississippi out in the sticks where houses are 2 or 3 miles away and it doesnt interfere with anyone. A TS "Sweet Sixteen" is 1600 watts and about as big as you can put in a vehicle unless you have a diesel pickup with 2 or 3 high power alternators. I have been around a lot of amps over the years out in the country side. The largest I ever had was 750 watts and it dimmed my low beams with a 100 amp alternator that put out 100 amps at 1500 rpm. I used it on the road in west Texas 30 years ago from 8PM until I stopped to sleep at 4AM.
The foto is a Texas Star Sweet Sixteen 1600 watt. The 1000 watt "Hot Plate" looks the same.
A vehicle could potentially do 15KW ERP though with the right antenna. I have my doubts they are using a high gain antenna but most CB antenna specs seem to be wattage, length and 'range'. Not useful for such calculations and I don't care to dig that much for specs.