Tube Suggestions Onix SP3

As a rookie to tubes, one of the Melody 5881s tubes on the Onix SP3 I recently purchased on Agon has gone pzzzsst and I'm clueless and suffering without good sound. I have to keep within a reasonable budget but at the same time have a solid replacement for resale value. Can the original Melody 5881's be had anywhere? Otherwise the tube shop I called said I need to replace all four, rather than only 1 and recommended the NOS Sylvannia 5881 at nearly $90 per. I'm tempted to just buy another used Onix if that's the case. (Just kidding) Help