Mobo, PSU, case, fans, none of them came with screws? Huh
You can look around your house for some screws. You can also probably pick up small ones at a local hardware store that'll work. The fans are made of soft-ish plastic so anything close to the same size should fit. All else fails, most places like newegg sell them too: http://www.newegg.com/Product/Product.aspx?Item=N82E16811998099&nm_mc=KNC-GoogleAdwords-PC&cm_mmc=KNC-GoogleAdwords-PC-_-pla-_-Case+Accessories-_-N82E16811998099&gclid=CNKJtdGX2sgCFQkuaQodqe0DkA&gclsrc=aw.ds