A prior post discussed the need for sellers to be certain that they decommissioned former oil tanks properly. In the Puget Sound region you might want to check out these folks for when you need this kind of work done:
Fuel Tank Installation (FTI): 206-244-8020
Filco: 206-547-8347
Tankwise: 206-937-3995
Links to each firm are available by clicking on the name of the business above. Some lenders will make a decommissioning certificate a requirement for funding a loan so be sure to do what you can to provide the proper documentation for your sale or purchase. Providing a copy to the escrow company can also help facilitate passing of documents from party to party within the transaction and some firms will accept payment from the closing agent rather than directly from the seller. Be sure to ask for your options when you call to set up service.