External Strainer

You should find something you can use here:

http://www.grainger.com/Grainger/hydraulic-filtration/hydraulics/ecatalog/N-c6m/No-0?op=search

You would be much better off installing your filter/strainer on the fluid return to the tank as the system will not be starved for fluid as could be the case when installing any sort of restriction on the suction side of a hydraulic system.

A very good option can be found in the link previously posted, its a Spin-On filter cartridge that is cheap and easy to maintain.

If you first clean your reservoir, then install the filter on the fluid return you will be fine on most cases.

I am no hydraulics expert, but if the system blocks nothing works, it does not matter where you put the filter/strainer.

A strainer I would put in the suction side of the pump to catch welders helmets and I would have thought that filters should always be downstream of the pump.