Decent triodes seem to be unreasonably expensive at present. OK, the likes of PX4, PX25, PX25A output triodes are bound to be pricey, more than £100 even for a used tube. Which is why most folks on sensible budgets use output pentodes (or kinkless tetrodes, KT66) with the more extravagant using them as triodes, as in the famous Williamson amplifier.
But... why pay more than a few quid for a used L63 (6J5G)? Here's an old idea that might save valve amp experimenters a few pounds - 6Q7G was designed as a first audio triode for wireless sets, sure it's also got two diodes, but you don't have to use them. When I bought my wartime Vortexion PAs one had a couple of 6Q7G the other used EF37A connected as triodes - see the datasheet that can be downloaded from the Virtual Valve Museum, Mullard give data for using this valve as triode or pentode.