Ad network with free trial ?

Ad network with free trial ?
Hi folks ! i'm looking for good ad netword with free trial (i mean, free click or free display...) so you can test the network... i've already tested bidvertiser but the quality of traffic i receive do not match my needs and their auto-refund program really burn your bank account... so anyone know one good ad network i can test for free ? I've searched on the interne, but i'm fed up of finding broken links...
Thanks for you helps guys/girls
Have a nice day !