Any good garden centre will have a small enough pump for the job with a small pond, however, as has already been stated, small pumps get blocked but you can take them apart and free the impeller. So it may be better for you to pump out most of the water but bucket out the sludge.
I tend to use very large pumps that deal with most things and unless you have a very large pond they are not worth investing several hundred pounds buying one for a small pond.