i have filehosting site how to sell adspace
can anyone suggest where to get direct advertisers
Try adsesne ( but read the terms - they might not like downloading sites - I assumed that is why you host files ).
Also for direct advertiser place an ad / banner where you place the text: but ads here. this will be linked to the contact page or to a page with prices.
Getting a direct advertiser is always difficult but you can leave this job for sites like buysellads or performancing ads who will find advertisers for your sites and this will save your time and effort.
Implementing Adsense onto your site will be your best choice. 
Check out other file hosting sites and you will be able to see a list of potential advertisers =)
AdSense probably won’t work that well on a downloading site, as there will be little text to match the ads against - leaving you with likely irrelevant ads. Top tier ad networks often don’t like download sites either as they often host copyrighted materials which is directly against the ToS of almost all ad networks.
Many file hosters make their profit from selling premium accounts to uploaders and/or downloaders for extra features and speed - so that might be something to look into.
use buysellads or any cpm advertising to earn revenue from such websites.