There are different sites you can visit to find holiday apartments. Note that legislation in Amsterdam has changed and short stay apartments should require a minimum stay of seven nights. Only if owners have a hotel permit, are B&B's, or are temporarily out of the country, they are allowed to rent out their apartments for less than seven nights, although many of them still do.
Amsterdam apartment prices might be a bit more expensive than in other European cities, but compared with hotel prices they will be relatively cheap. Be carefull though, there are more and more scams on the internet, Craigslist offers apartments for allmost unbelievable prices as do a lot of scam websites, so if an apartment almost costs less than nothing, it cannot be true!
You can find hotel as well as shortstay apartments at http://luxury-apartments-amsterdam.com , which is a site run by a Amsterdam company, owned by two Amsterdam apartment owners, including myself. If you have any questions, you can ask them through our site.
With regards,
Dries