I was at Albrecht's Nurseries in Narberth yesterday and was rather surprised to see such a wide variety of organic lawn care products. However, they are one of the more expensive nurseries around. I wish I knew a source for less expensive items.
Waterloo always has a big sale on Memorial Day weekend, so if they carry what you want and if it goes on sale,you might do well there.
I guess Home Depot hasn't gone organic yet, although I didn't look there.
Good luck with your search.

Try Gardens Alive! on the Web. Shipping is pricey, that's the only thing, but they issue $25 coupons from time to time for purchases of $50 or more. That covers shipping. Another option is the national franchise called Natural Lawn of America if you don't feel like shopping and doing the work yourself. There's a local franchise in Montgomery county. They charge about $400 per year for 8 treatments. They are "organic based" but do use an herbicide for spot treating weeds unless you tell them not to. Their approach is to get the soil healthy by using all organic fertilizers, aearation, and overseeding with grass that does well for your plot. After 4 years, my lawn looks pretty good. I've cut out the weed treatment completely and the grass is thick and healthy enough to minimize weeds on its own.

Probably a little late, but...

Pughtown Agway on Rt 100 near Pottstown (about 30 min away) has Organic Epsoma Corn Gluten Meal and Rock Phosphate, but not in super large quanities. Their price are better than anyone in the area, but Epsoma products are still not cheap. It think they have bulk corn gluten meal, but it is not organic. If you google Pughtown Agway, you'll see the address and phone.

Bellatrix

Bella, I don't think Pughtown is considered the Main Line ;)

I saw some organic items at the K-Mart in Strafford. I also second the suggestion of Gardens Alive.