I suggest you do a Google search of clean boosters. When you find examples, look at what drives the circuit, they'll list it in the specifications. You don't want something that lists overdrive or diode clipping in the features. Even classic linear boosts are going to color your tone, adding a slight darkness and warmth to the sound.
Mosfets are used in most of the good clean boosts out there. Keeley's Katana, BBE's Boosta Grande, ZVex's Super Hard On, or Fulltone's Gt-500 all have good clean boost available.
Used on the 'pushed' setting on your clean channel, you'll be surprised at how much gain is available. In fact, I sometimes use the clean channel on pushed with either a clean boost or a good overdrive with the gain turned way down and the level knob cranked to get a decent passable Marshall JCM-800 kind of sound. I use this configuration to play Iron Maiden, Judas Priest, AC/DC, Zeppelin, etc.