There's an extension cunningly called "PM Search" for phpBB if the webmaster fancies installing it, used it myself. Failing that, the tedious way is to scroll to the bottom of the page, export each page to a CSV, join them*, open in notepad or Excel and use the built in search, upside is you also get a handy archive.
*open a command prompt, "cd documents" or wherever you saved them, type something like: copy data.csv + data(1).csv + data(2).csv pmhistory.csv
Current fleet: Triumph Dolomite Sprint '75, Daihatsu Fourtrak, Honda CG125, Yamaha Fazer 600, Shetland 570
Disposal fleet: Golf GTi 16v MK3 Anniversary
Past fleet: Triumph 2000, Lancia Beta Coupe, BL Mini Clubman, Austin Metro, Vauxhall Cavalier MK1 & MK2, Renault 18 D, Rover 216 GSI, Honda Accord (most expensive car purchase, hated it on pickup from dealer, was made out of magnetic metal as only car I've ever been crashed into, 4 times), Golf GTi MK3 16v x 3