CAT SL1 MK 2 Reference Preamp Tube Replacement

Can anyone recommend an reasonable but excellent sounding tube for a CAT SL1 Mk 2 Reference preamp. I am considering selling it and 4 of the tubes need to be replaced. If possible please provide the best place to buy them. Thanks very much.

Talk to Kevin at Upscale Audio. He was helpful in my tube rolling for my CAT Sig.
Knows his stuff when it comes to CAT.