They're not expensive... you can get basic ones for $10. Make sure when installing it that you correctly set the jumpers (unless it is PNP, in which case you can ignore this) so as not to use the address of the built-in port or your modem, or an IRQ that's already in use. COM1 uses IRQ4, COM2 IRQ3. The default IRQs for COM3 is IRQ4, and for COM4 IRQ3 but that will not work right if you're using them for COM1 or COM2.

You'll probaby want to use IRQ5, IRQ7 or IRQ9... whichever one isn't being used by your sound card.