An electric jug will do most things better than an immersion heater in your case.
If you can to minimise water used be sure that the coil of the element face downwards towards bottom of the jug. As long as the water is say 5mm above the parts of the element that get hot it will work well. You may be able to have a small part of the hot portion above water as it will be "heat sunk" by the underwater part.
If you really want an immersion heater and cannot find one you can make one from a jug + element. In many cases you can unscrew the element and remove it then add part of the cord socket to it. How easy this is will vary with design. Dropping the element plus cord into water when live is "not recommended".
Some (hopefully few) immersion heaters are so flimsy as to be dangerous.
Be aware of what aspects of use are potentially dangerous or lethal.
Semi-related: I stayed in a Chinese hotel where the hot water temperature was so low that a decent bath could not be had (maybe 30C?). I like to soak in a hot bath when facilities allow to offset the effects of overdoing things the rest of the time. I bought an immersion heater and used it to raise the temperature of a bathful of hot water to something more acceptable. (DO NOT introduce ANY part of your body to bath water while doing this!!!. ). Inspection of the heater showed that the chance of it not electrocuting somebody long term in its intended role was "low". I dealt with the heater with extreme prejudice" before binning it on departure. 'Let no man heat with thee hereafter'.