they are normally 20x20mm or so. I have seen any smaller ones advertised except for some corian ones on ebay. How many are you using, the cheapest way of getting them is to cut them yourself from boards or offcuts and you can cut them at what size you want. If you want a selection of woods, however it is going to be a pain for somebody to cut a couple of smaller blanks from several species rather than have a session sawing and grabbing a few from their standard stock for each order.
for most species I doubt that there would be much saving.
As Marcros said if you have a bandsaw then cut your own, there's plenty of wood available if you look apart from collecting sawn locally or from the garden where you can cut up oversize and it will dry very quickly, doesn't matter if it moves a bit.
Examples of dried wood are:
* Old furniture ( chair and table legs but also there's a lot of more modern thrown away stuff as well made from Indian rosewood which can be nice, I was gifted some 16mm thick panels from bedside cabinets)
* Chopping boards in olive and bamboo there's nice stuff usually in places like TK Max. Try crosscutting some for different effects.
* Offcuts from flooring companies also odd sample lengths cheap, you can even send on line for samples of many woods for the cost of postage so flooring and worktops.
* Corian and similar solid surface offcuts and sink / hob cutouts are usually skipped, you'll probably find some of those on the Dragonville Ind Est in Durham.
* Small joinery workshops and window / door manufacturers burn or skip the offcuts.
* Hardwood pallets, you would be surprised what you can get and after all you're looking for wild gran if possible.
* Old broom and shovel handles etc sometimes are worth looking at.
The list is almost endless but failing that if you want some therapy and likely spend some money go and visit Vince at The Turners Workshop Home | The Turners Workshop You will need to arrange in advance as it's not really a shop and his hours vary but he's a really nice guy with enough varied stock to keep you interested, not everything is listed on his website and it's well worth the trip just north of Newcastle. Spare time and you coud also pop into Axminster just 10 minutes from him.
I'm not too far up the road in Northumberland.