Honeywell F51B Series Parts
Some parts are still available for this model but the unit has been discontinued. If you wish to replace the entire unit order an F300 model. You will find a link to two of them below.
Honeywell 4074EHG Cell Contact Repair Kit
4074EHGOn Sale $16.74
Custom HEAVY DUTY Size Prefilter
CUSTOMPREFILTEROn Sale $80.40