Does anyone know of a lender in Washington who will do a home equity line of credit (HELOC) on an investment property ?

FEMA actually needs to visibly verify the presence of the flooded appliances to give you credit for it. 

Safe Credit Union, US Bank, El Dorado Savings, Golden Pacific Bank.

Also, any CPA recommendations and/or tips on keeping track of expenses/documents?

Rates well vary widely depending on credit and down payment amount.

I've had a client purchase multiple properties by securing a business line of credit against their personal residence (which was free and clear) and use that as the downpayment for several rental properties. 

Thank you for any advice, tips, feedback, comments, etc!

I also have a $90k business line of credit (BLOC) for when the HELOC runs dry.  

Do you have any tips, websites or general rule of thumb?

Regardless, what I'm trying to say is that you don't have to follow the same path but talk to several other lenders as there are options with smaller credit unions if you're strictly looking for buy and hold without having to do rehab.