Well, the best thinner for Tamiya acrylics is… Tamiya’s own thinner!
But if you enjoy mixing stuff, here is a cheap recipe I found a while ago on the Internet. I tried it several times and it works beautifully.
Water (25%) + isopropyl alcohol (75%) + a few drops of food grade glycerine.
The first time I tried it, I was able to prime two 1:35 tanks – with constant pauses (I even had to take a phone call that lasted a few minutes) – and my Iwata never had any kind of clogging during the 40 minutes of spraying session. I have recently filled a couple of one-liter bottles with the stuff spending just a few euros.
————————————-
Another kind of thinner that works with Tamiya (I personally never tried it) is the thinner used for lacquer paints
Resources for scale modellers 