Depends what you consider inexpensive. I'd say you'll need to spend a minimum of $400 to $500 to do it right. Don't try to skimp on tubes or buy from a dealer without a solid reputation or you may regret it.
You'll need two closely matched 12AX7's & two matched 6922's.
I have a variety of tubes that I have used with my Calypso's.
For the 12ax7 tubes, I prefer Mullard CV4004 Platinums (available from Upscale Audio) or Telefunken 12ax7 smooth or ribbed plates. For the 6922's, i like "white" label Amperex or Phillips SQ's.
In addition to Upscale Audio (good prices, but kind of snooty), I like Brent Jessee's tubes and also Vintage Tube Services (be prepared to wait 3 weeks to get your tubes if you buy from Vintage Tube Services). All of these suppliers can be found via Google and they are reputable.