In my opinion it's not worth making your own underglazes. Commercial underglazes are plentiful, work very well, and can be purchased relatively cheaply, especially when they go on sale. In addition to all the testing that will be required to get a homemade underglaze to work the way you want it to, it will take numerous rounds of testing to work out the colors and a lot of wasted money buying stains that won't be right. Plus underglazes are saturated with colorants, and it may not be all that much cheaper to make your own once you factor in the cost of the stains, especially reds and oranges.
I use Speedball brand underglazes. They are quite cheap and work very well. Sign up for the newsletter at www.Clay-King.com and watch for their sales. Several times a year they'll have Speedball products at 40%-50% off, which gets them down to about $6 per pint or less, and they usually need to be watered down a bit so it's a really good deal.
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