There is always Reverb.

Blenheim drums in England, althought he is currently relocating so no open for business. 

https://blenheimdrums.yolasite.com/

He has a youtube channel called the drum fettler. He has a couple of videos where a friend of his, James Marston, who used to work at the Premier factory assembling pedals shows the restoration process so that person might be able to point you in the right direction. James appears to be on facebook.

https://youtu.be/la8XaO0r6Jo?si=DEpAHBLNwQrvQW9_&t=252

Thanks very much. Yes, I've found those YouTube videos and also Blenheim drums but doesn't seem to be any other sources. I'll keep checking eBay and Reverb. 
I've recently bought a second 252 for my other bass drum and despite stripping it down and rebuilding, just can't get it to work as well as the other one. Seems sluggish and unresponsive. Maybe it's the spring? 

This site and Drum Forum have always been helpful. Sometimes I've gotten a better price on right here.

Really can only be a few things. The main shaft should rotate smoothly and have no noticable play.

Make sure the footplate hinge operates smoothly.

The crankshaft/connecting rod assembly should move freely with little play. The connecting rod is made out of nylon and can wear where it attaches to the crank. Also make sure the surface of the connecting rod that fits inside the spring is smooth so it doesn't bind on the spring. 

The spring has compressed and lost it strength.  If you take the spring out of your good pedal, compare the uncompressed length. If the spring out of the problem pedal is shorter then it has compressed. You can restretch it by putting a tightly fitting dowel or metal rod in it and grab each end with pliers or vicegrips and slowly restretch it slightly longer than the good one.  

It also might be possible to find an appropriate replacement spring at a  well stocked hardware store.

Also don't over grease the spring or any components with too heavy a grease. That can also slow things down.