Most of us buy the blocks of floral foam and a craft store or Walmart's craft/floral department and simply slice off slabs of appropriate thickness.   A few bucks will buy enough to last a season or two. 
I nailed & glued up simple DIY thee sided open top-and-ended "box" with a saw kerf square across as kind of a simple miter box. The individual blocks of foam fit in snugly. I stared making the slices with a sharp bread slicing knife but now use one of those reciprocating electric bread slicing knives I found at Goodwill.  i can make enough for pretty much a season in half an hour or less.  Addendum: FWIW I find that those cheap "reusable/disposable" plastic left-over savers in the grocery stores are good for storing the slices in.  the one sized for a 1lb stick of butter works well for me, and it keeps the slices from crumbling and crudding up my shooting boxes.

Thank you both. How thick should the f. f. be cut and are the wads punched with a hammer-type wad punch or must they be cut by some means?

How thick?

You want the dry floral foam and not the wet variety.

The foam can be used to accomplish 2 things.   
If just pushed down onto the upturned mouth of a primed and charged case---like a reverse cookie cutter----- it will help keep powder from spilling into the action when slipping a charged cartridge into the chamber after breech-seating a bullet.

Or if you have a case with straight interior walls it can be lightly pressed down until it is ALMOST in contact with the powder charge.  this holds the powder back for a more uniform ignition and keeps you from spilling it into the action as above.   obviously this will not work well with tapered or bottlenecked cases 
If considering this you might want to search and read up on "over-powder wads"

  I and everybody I know uses the wet type.

Interesting Jeff, must be a East coast West coast thing everybody out here uses the dry. OkeyMaple go with what the East coast fellows are using, them fellows out that way are shooting some real impressive group scores. It would best to copy everything they are doing so wet it is.

Thanks for the East Coast headsup Jeff it is highly appreciated on my behalf.