Yes, there are hardwired transformers available. Just google "hardwired lighting transformers"

Google WAC lighting.transformer.

You can do this a couple of different ways. 1. A regular lighting dimmer for your plug-in power supply (what you are calling a transformer) with a DC power cord going to your lights. 2. A DC dimmer (rheostat) mounted with the lights (could be hidden under the cabinets right next to the lights). A plug-in power supply may hum or buzz if not designed to be run at anything less than full power, however. A ceiling fan type dimmer could be used, but those are generally more expensive. The plug in power supply (a.k.a. wall wart) may also be unsightly, but could also be placed under the cabinets, requiring a receptacle in that location as well and then a requisite dimmer. If you already have a plug in power power supply and the lights hooked up, I would go with a rheostat and mount it under the cabinets where you could just reach under and give it a quick twist when you wanted to dim the lights or turn them completely off. Keep in mind, though, that the rheostat will be a power vampire whereas a traditional dimmer for the wall receptacle would reduce the power use or turn it completely off when the lights were not needed.

120 volt receptacles are not to be placed on a dimmer.