Would this be ok?

So my case(fractal design core 1000) has its psu mount on the top. Now most PSU's have the fan on the top. Would it be ok just to put it in or if not is it possible to flip it upside down? Or is there a 500 watt psu with the fan on the bottom or back or something?

The fan placement doesn't matter a whole lot.  I place all of my cpus so that they face the rest of the case.  (e.g. if bottom mounted the fan points up and vice versa)

Top mounted PSU should always have the fan pointed down. Bottom mounted PSU can face either up or down depending if there is a filter underneath.

Most PSU are deisgned to allowed to be mounted both at top and bottom. Just flip your PSU to face down.