purolator makes one. also you can get an inline filter from ford. its really good. you splice it inline with the cooler lines. we put them in any time we replaced a tranny at the dealer.
The internal OE filter filters out only the 'BIG" stuff, the magnet in the pan does the rest. We use a 3/8" inline unit made by Magnefine which has a magnet and a filter for fine non-magnetics all in one unit. Costs $17 at the tranny parts store. Change the Mercon V every year and the filter every 2 years and add a plate style cooler.