Leitz Tooling on an estate facing the New Cut, just round the bend from the dogs' home.*
I know that sounds like a music hall song, but they're nice people and have a quick turnaround. They do my circular blades, and the prices are reasonable. Morally certain they do planer knives too.
If not them then SWEMCO in Brislington: specialists in guillotine knives (ten feet long or more). They do the ones for my ancient printers guillotine, which are a mere two foot long. Their workshop is, frankly, terrifying, but they'll only let you look in from the doorway.
E.
*If you come from the Temple Meads gyratory, along the Bath Road, turn left at the first bridge over the New Cut, left at the T jct. they're in the first estate on the left (quite small but plenty of parking). (edit) Ferry Steps Ind Est. off Albert Road. You can get there from the inner ring road easily too, over the new St. Philip's Causeway bridge then turn right at the Avonmeads roundabout.