Any alternative to 6GK5 tube?

I am interested if there is any alternative/substitute to a 5GK6 vacuum tube that i can experiment with.The choice of 6GK5 tubes is very limited online,basically GE only and some suspicious RCAs on Ebay.I couldn't find any 6GK5s in the Tube Store or Music Direct and Parts Express sites either.I need them for CJ Premier Ten and CJ wants $ 107 plus shipping for them.Any other tubes that i can substitute in place of 6GK5s?Thank You for your replies.

Mike is a good dealer.
Search on Ebay for 6GK5 and pick up some of the Japanese Hitachis for peanuts. When I had a CJ Premier 14 I preferred them BY FAR to anything else I had used -- from GEs to Mullards. Good luck, Dave

prices start $21/pair at: