I'm considering playing with different springs in my Tx, and wonder if anyone else has, or knows a source.

Why ? I run my sticks with a fairly soft tension; and I also have more than one Tx. What I've found, is that on some Txs, when you back the pre-load off enough so the sticks aren't so heavy at the extremes; the centring becomes non-existent. [Another Tx will centre fine, without being heavy at the extremes.]

A possible solution would be to use softer, or longer, springs. This would give enough pre-load to centre; without a massive increase in tension at the extremes.

I also fancy trying soft stick tension, with a positive centring click.

The Txs in question are DX8s; and part of the problem is inconsistency between different Txs; or even between axes on the same gimbal. I'd consider springs from other Txs. It's not too bad getting the gimbal/stick assembly out.

BTW, I came across one gimbal where the spring tension adjuster wouldn't wind in fully. The adjuster screw fell out first. The adjuster was fouling on a steel pin, IIRC. I filed the plastic a bit to fix this.

A bit of theory

The spring set-up has two distinct properties: pre-load; and stiffness.

The spring tension is the pre-load plus extension times stiffness.

The centring is provided by the pre-load.

For a given pre-load; the only way to adjust the tension at the extremes is to change the spring stiffness.