Well, there are tables that gives you the coefficients of the transfer function, here u can find up to 10th order:
**broken link removed**
other thing u can do is to use matlab to make the filter, matlab gives you the order of the filter and the coefficients, you only give it the atenatuion you eant and your cutoff frecuencies....
this are the commands:
buttord (To get the order of the filter u should use)
butter (To get the coefficients of the filter)
I think those are the commands, i am pretty sure about it, check how to use them on the matlab help.