I play various percussion, and was keen to get a washboard, but not to pay the £45 or so that they charge for the newly manufactured ones (never been anywhere near a load of washing!) I was lucky enough to get one in an antique shop in the Scottish Borders the other week for a mere £5 (didn't even try to beat the man down at that price!) it's metal one side, glass the other, and if anything, I prefer the glass side sound as it's crisper. I got thimbles in John Lewis in Edinburgh (metal or nothing) - £1.50.p each, so I bought enough for 3 fingers of each hand - so thimbles cost more than the washboard!
I did see a "skiffle band" the other year, where the washboard player used a guitar slide in one hand and a plectrum in the other: sorry, but it just didn't work!