The spectrum from 40 to 47 MHz is used by commercial and state agencies in North America which means that there is a certain amount of equipment available for the band.
Pasternack Enterprises is one such provider of equipment and they sell a mobile antenna which covers 40 to 47 MHz.
It would seem that this is a full size quarter wave antenna and at 40 MHz would be just under 2 metres in length!
The antenna costs in the region of $60 with the associated mount costing about $50.
They also have distributors in the UK and Europe.
1) Pasternack Mobile Antenna
2) Associated base