https://tkbtrading.com/TKB is the cheapest out there for oxides and ultramarines but high quality.
https://justpigments.com/cartJust Pigments is a good place for mica in large quantities. They offer free shipping for $10 or more orders.
I use white mica instead of titanium dioxide for lightening my colors. Also synthetic micas are the brightest micas that you will find but they are expensive. However they are well worth the price because they blend so well with other pigments.
I use TD for some soap colors and I get it from Amazon. If you do some online research you can find some different shades of oxides, like tan and creams, brighter reds.
Hydrated chromium green and chromium green are good colors to start with for different green and blue colors.
I also recommend manganese violet for purple and red colors.