Just to be clear, is that 45V 100A spec the motor's stall current? Or loaded current? Most motors are rated for a particular current at a particular torque load and rpm. If your 100A spec is the loaded/running current, I'm afraid you'll need far more than 100A when it turns on (since any motor that is stopped is starting in a stalled state) or any time it is loaded more than the rated torque @ rpm.
If you want to be sure, estimating stall current is relatively simple, at least if it is a brushed DC motor. Disconnect it from anything else, and measure the resistance between the power leads. Now divid 45V by that resistance. That is approximately the stall current.
The reason you can't find any suitable buck converters (keeping in mind that I am talking about 45V @ 100A, if that is all you really need) is because they are not practical at that voltage and current for the most part. It is certainly possible to design such a converter using multiphase controllers, but there is a reason no one has actually done that. The losses are unfavorable compared to other methods while the cost is higher. So you pay more for less.
Buck converters must store a lot of energy in a magnetic field around an inductor. Some is transferred directly to the load, but some is stored as well. This requires big, heavy, and expensive magnetic cores. You can reduce the needed size by increasing the frequency, but this will increase the losses significantly. 4500W is far outside the power range where buck converters are typically useful (unless they are very high voltage, like 600V+). 4500W requires something like a push-pull or full bridge converter, but you will be hard pressed to find one with such a strange set of voltage ranges at such a high current.
The real issue is that you're trying to solve an easily solved problem with an extremely difficult solution. The easy way to solve this problem is to simply power the right motor from the right power source. You give a voltage range, so I assume that means you are powering it from batteries. So remove some series cells to get a lower voltage (slightly above the rated voltage is probably ok though you might reduce the motor's life somewhat. If that is a concern, then you should be derating the motor anyway and powering it with a slightly lower voltage that it is rated for). Yes, you'll get well below 45V as the batteries are discharged, but such is life. If this is a vehicle, and you had one powered with a buck converter and the higher voltage, and one with a matched voltage (45V-38V), the one with the lower voltage will travel a further distance simply because the extra energy will be more than consumed as losses in the buck converter for the higher voltage one.
Simply put, even if you could find a suitable DC/DC converter (which I think is unlikely), it will almost certainly cost more than simply buying the correct motor and/or power source so they match, so do that instead. And derate the motor so it will provide enough power even at the lowest voltage it might be powered from but will have no issues with the higher voltage as well.
That is the solution to your problem and is far better and easier than using a buck converter.