^^^^^ all that.
parts express and mcm electronics are good sources for "just need a speaker" situations. find something cheap, with 4 or 5mm xmax, with a frequency response like 40 or 50 to 3000 or so, over 95 dB, and it might not really sound too bad. pick up the free win ISD and measure the inside of the box. model away until something turns up a fairly horizontal spl plot.
is it an open back cab? if so, just chuck whatever in it: it probably won't sound too awesome no matter what, but it'll work.
is there a connector for an external cab on the amp? if there is, and it says something like "8 ohm minimum" right above or below this connector, you could disconnect and tape off the speaker leads and use (probably) a 4 ohm bass cab (if it says 16 ohm minimum, it probably would have to be an 8 ohm cab). i'd bet this would sound better than any other option (including the one I just mentioned). good luck!