In the UK I'd give Watford Valves a call. I've had nothing but great advice and customer service from them, particularly Derek, who knows practically everything about valve amps - it's a long conversation but fascinating.
[url="http://www.watfordvalves.com/index.asp"]http://www.watfordvalves.com/index.asp[/url]
Most preamp valves of this type (12AX7, 5751, ECC83 etc…) tend to be a uniform size and have a standard 9 pin array at the bottom, and are all skinny valves .