You do need the washers.
You can get identical, steel washers to what I received with a set of NISI rims 25+ years ago in a 100-pack at your local Home Depot (or, equivalent) store. Probably, about $3.00US for a blister pack of 100.
I don't know the "size"; but, a few years ago when I was thinking about buying some more, I brought one which was removed from a rim, and there they were.
Depending on your spoke calculator, I don't think you have to change the ERD -- probably, 616.0 ... I always round-down on the spoke length.
Al Feng Albuquerque, NM
On Wed, 25 Oct 2006 07:05:09 -0700 (PDT) Jerome & Elizabeth Moos
<jerrymoos@sbcglobal.net> writes:
> I am currently, for the first time in many years, faced with building
> a wheel using a rim with no eyelets, i.e. an alloy tubular rim with
> unreinforced holes. When building rims without eyelets, does one
> always need to use a washer inside the rim between the rim and the
> nipple? Does anyone stiil sell these, or does one simply find an
> appropriate size washer at the hardware store? Obviously, if using
> washers, one needs to allow for them when selecting spoke length.
>
> Regards,
>
> Jerry Moos
> Big Spring, Republic of Texas