You can still purchase a license for FontLab Studio 5 directly from the developer, though they strongly recommend upgrading to the modern successor, .
Upgrade from other font editors (FontLab Studio 5, FontLab VI, TypeTool, Fontographer, RoboFont, Glyphs) for US$ 459. Serial Number Fontlab Studio 5 Mac
If you have lost your original serial number, you should contact orders@fontlab.com with your original purchase details to have it resent. Activation & Installation on Modern Mac Systems You can still purchase a license for FontLab