Mr. Color Thinner (MCT) and Mr. Leveling Thinner (MLT) are two lacquer thinners made by Gunze Sangyo. You can purchase these thinners from a number of stores like Amazon and Model Grade.
You can consider Mr. Color Thinner to be a ‘generic’ lacquer thinner. It works well with Mr. Color brand paints and Gaia Notes colors. MCT can be purchased in 110ml, 250ml and 400ml bottles. The 400ml bottles are the most common size you will find for sale in the US (in my experience).
Mr. Leveling Thinner is slightly different than MCT. The thinner in MLT is blended with a retarder; the retarder slows down the drying time of paint. This allows the paint to spread out more evenly, leaving a smoother and glossier finish. MLT is also the preferred thinner of modelers in very hot and dry climates. I like to use MLT with Mr. Color GX brand paints and with Gaia Notes ‘Starbright’ line of metallic paints. In both cases the thinner allows the paint to dry extremely smoothly leaving a brilliant gloss and fantastic looking metals. MCT can be purchased in 110ml and 400ml bottles; on average I have seen the 400ml bottle of MLT run $2-3 dollars more than an equivalent sized bottle of MCT.
I recommend using MCT as your ‘standard’ thinner and only using MLT for high gloss and metallic paints. You can use MLT for everything, but I personally think it is unnecessary.