The next task was to find somewhere to buy the tubes themselves. They are harder to find than you may think, many places have no stock or have very small quantities in stock and therefore are extremely expensive.
Places like eBay are your best bet when it comes to finding some ‘new old stock*’ IN-18 tubes. I managed to pick up 6 tubes at a reasonable price from the Ukraine.
*Because the tubes are no longer produced anymore, people sell new old stock. This just means that they are new, never used tubes but were produced years ago.