I want to buy adjustable ring bases/ ring blanks for making custom rings. I need the kind that have a flat metal top for gluing charms and things on. Does anybody know where I could buy these?

That is a really nice idea, I'm afraid I just don't have the patience to make jewelry, although I am trying my hand at hair clip making!
One place that is a definate MUST for all types of crafting supplies is Ebay!  I have been getting tons of great deals on my crafting needs!
One in particular is an auction for 10 silver blanks at a buy it now price of $3.74 with a shipping price of $3.74!
specifics are:
Up for your jewelry-making and creative inspiration are (10) pieces of adjustable silver-tone ring pads.  Glue pad measures approximately 10mm across.
These are perfect for gluing on beads, buttons, typewriter keys, fabric covered buttons,  etc.
Disk is textured to make the attachment more secure. Although this finding is adjustable to almost any size, once it is significantly expanded, it is more difficult to contract.

Another deal is a grom of 10 silvertone with a buy it now price of $1.09 with $3,79 shipping!
specifics are:
Quickly and easily make your own beautiful rings by bonding any flat component onto the pad.
Perfect for larger components and cabochons, dichroic glass, crystals, beads, shells, buttons, vintage typewriter keys, altered art and anything else your creative mind comes up with
Premium quality, delicated arts and crafts

There is also a great bulk deal of 100 with a buy it now price of $35.00 with $4.95 shipping!

Try Fire Mountain Gems and Beads (http://www.firemountaingems.com). They've got everything in the world a jewelry maker could want or need.

Have you tried http://www.PerfectBead.com? They have dozens of styes, ranging from 50 cents a pie ce to over a dollar.