AMEN on Eheim. Quiet and efficient.
There's a guy out of Houston that sells Eheim filters on Ebay. I got my 2213 (which is big enough for your 50) w/filter media from him for about $75.00 using the ebay "buy it now" feature. Just do a search for Eheim and one of his should pop-up. If not, just check back every day and before long he'll put one out there. 8)