Hi Jeff,
The general consensus is that the best thinner for tamiya paints is their own proprietary thinner.
Now having said that, their thinner seems to be primarily isopropyl alcohol plus a retarder. Many members here who use Tamiya acryics thin with isopropyl.
I have used methylated spirits, which is essentially ethyl alcohol, fairly successfully. (more than likely, you have a bottle under your kitchen sink) I'd have to say that if you use metho, retarder is essential as it evaporates very quickly. Acrylic retarder may be found at most artists' supplies stores and a bottle will last you for many years.