I've been turning for only about 2 years so my experience acquiring bowl blanks is all over the place. I have gotten bowl blanks . . .
-Purchasing them from a Woodcraft Store - some are drier than others but mostly dry enough to do final turning without further drying.
-Buying bowl blanks online-again dried enough to just turn without further drying
-Purchased some partially dried blanks from another turner who acquires and buys logs which he saws up into blanks
-Free wood from a Woodworking Club fellow member which isn't dried and must be coated so that it doesn't just split apart.
-Glued up blanks from scrap hardwood in my shop-Glued in various configurations. You can get some interesting designs by gluing up blanks orienting the grain in various directions in each layer. You still see glue lines but it becomes part of the design.
As for drying...
-Any wood that is still green or not dry enough I coat with AnchorSeal (a water based sealer that just gets brushed on); at least all end grain but safer if the whole blank is coated. That keeps the moisture in till you're ready to turn the bowl.
-I purchased a Dry Fast kit from Rockler awhile back. It includes a pail with a sealed cover and silica beads which absorb moisture and can be regenerated in the oven after they have absorbed moisture (turn pink). It works well with green wood - rough turning a bowl, then submersion in the beads for a couple of days, then final turning. However, the pail will only work for bowls about 9" or less in diameter. The concept will work for larger bowls but you will need your own container and more beads. Anyway, so far, I've had excellent success with it. However, if you're looking for a system which will handle a large volume of bowls, this isn't it.
There are a number of online stores that sell both green and kiln dried blanks. However, you are, more or less, stuck with whatever they send you in the wood and size you ordered. One of them, turningblanks.net seems to be fairly reasonable and the blank quality isn't bad.