One can never have too many big letters laying around the house.
https://threepotatofourshop.com/index.phpthis place sells all sorts of cool stuff. their inventory is always changing. i know a couple months back they actually did have big letters that lit up. keep an eye out—they get new stuff all the time.
Peters, you can actually buy the letter of your choice from Urban Outfitter's Apartment section. I think they're $178/piece. Of course they are mass produced and a bit flimsier than an actual sign letter, but thought I'd mention. Hopefully this link works...http://www.urbanoutfitters.com/urban/catalog/productdetail.jsp?_dyncharset=ISO88591&navAction=jump&id=16815151&search=true&isProduct=true&parentid=SEARCH+RESULTS&color=24