For example we ask for trade references that they use like suppliers of materials, and we call to verify they pay their bills on time.

Systematically blaming the contractant is lack of leadership, and as a business we should not let room to our suppliers/contractors to take advantage. 

Wondering if anyone can recommend door & window suppliers in the Seattle area?

Visit your local ABC Roofing Supply and show them your photosAsk them for the local COMMERCIAL sales reps for the TPO and PVC suppliers ... you want to have them all come to your building and discuss with you their products, warranties, and their "CERTIFIED" roofers; the certified roofers can sell warranties - even if you don't BUY the extra warranty, it's good to know which companies are certified (which required training). 

It doesn't take a genius to watch how contractors, suppliers, etc. treat the manager/principal.

Maybe start out by signing up for mailing lists for the supplier stores you'll frequent, including the manufacturers' sites of materials you like.

Hand tools you buy like hammers, chisels, wrenches start to rust before you walk out of the stores.I was talking to a supplier to Home Depot two years ago and he told me that in our areas or regions, the average shelf life of an item at a Home Depot store was 8.1 minutes or some such crazy number.

All costs from suppliers and vendors 4.  

Do you have a good supplier, that makes quality cabinets with quality material?

Hi,I think a general contractor has reasonable price can save you money by negotiating effectively with subs and suppliers, and managing the work efficiently to avoid extra costs.