Some ornamental grasses may have seed almost as small as dust and can be purchased in bulk. The people at Gardenweb's Ornamental Grasses forum can give you links to good sources.

Nice I'll post there too. Basically I want something that looks like this after a week or two - http://files.getdropbox.com/u/455762/ex_irish_moss.jpg

But at the same time is cheap to buy in bulk.

I haven't explained my situation well... let me start over. Basically I'm working on this art project where we need to arrange seeds into different patterns and then film the seeds sprouting / creating a pattern / drawing. This will really work with a lot of different kinds of seeds but the tricky part is I need to keep the detail really fine even when the whole layout is going to be massive. Hence after trying a bunch of seeds I've found Irish Moss to work nicely, it has tiny little seeds that produce tiny little sprouts (hence great pattern detail). Only problem is buying Irish Moss seeds gets pricey really quickly... So basically I want cheap tiny seeds that will grow like this - http://files.getdropbox.com/u/455762/ex_irish_moss.jpg
Any help would be AWESOME!

Rye grass will perform like that. It will also germinate very quickly. Keep it clipped very short as it germinates and it will get denser and denser and finer.

It's used to over-seed golf course greens during the winter, in the south; that should tell you how fine-textured and thick it can become in a very short time.

I covered an old tie with rye grass seed as a joke one year.

The grass can't be cut. It has to grow undisturbed. Will rye grass grow super short and dense if uncut? I looked up picture of it and it seems to want to get pretty tall.

Any other ideas?

Mother of thyme would fill the bill perfectly.

Lainey

Here is a link that might be useful: Mother of thyme seeds