I'd just go to any large department store. (Nordstrom, Neimans, Macys, - any. Maybe even JC Penney or Marshalls or TJ maxx)

In the last week I've bought 2 cloche-type hats - one at Macy's that looks very 1920's-ish, and one at Nordstrom's

Sounds like good advice from Janisj. I do not have any suggestions but hope you post here afterward as I am a Downton diehard fan!

Thanks Janisj,
Not sure I want to pay Nordstrom or Neiman prices for a one-time event but will be trying some of the others this weekend. Didn't want to run all over but looks like I'll have to.
I also found this as a fall back which could work with expedited shipping: http://www.victoriantradingco.com/store/catalogimages/16q/i5674.html

If you have a "SAKS Off 5th" check out their hat selections. I was in a similar predicament as you and found one there.

I would go as a maid.

I really would try Macy's. All their winter hats are on clearance and I picked up a really cute cloche w/ sort of a feather feather medallion on the side for less than $16. very 20's looking.

Well Janisj I did find a great hat at Macy's today--thank you for the suggestion. It's a dark grey cloche with black lace trim. I could have bought a cheaper hat at Penneys but I may wear this one again. This maybe the same hat: http://www.zappos.com/jessica-simpson-lace-band-cloche
I've got a longish black dress and some shoes that are sort of 20s styled and long grey beads so I'm set. This should be fun.

Yes, I was going to remind you to wear your PEARLS!

Great! Glad it worked out. It's cute and looks wearable even when you aren't in character. When I bought mine I saw what seems to be the hat you bought only in green.

Mine is close to this only in a pretty, sort of light rust brown http://www1.macys.com/shop/product/collection-xiix-hat-feather-cloche?ID=695283