If you do not wish to make your own out of piano wire you can buy one here
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Charlie

I have made a couple for the 43 Spanish.  Not hard at all to do.  Search the web for a tutorial on it.

I used one of the spring kits from Rolling block parts on my 45/70 and I was quite happy with the results.

I wasn't going to reply to this, because it seems self-serving.  But, after the suggestion to google how to reduce the trigger pull on a rolling block (I did that search) I thought I should once again put out some good solid information to help counter some of the really bad advice (in my opinion) that shows up on google.

There is only one thing that is needed to reduce the pull on an undamaged rolling block to an acceptable level.  That one thing is to replace the trigger spring.  If that doesn't reduce it adequately, the hammer or trigger sear surfaces are damaged or have been improperly stoned, and will require corrective work.   

There are several ways to replace the trigger spring.  One way, is thin and narrow the original spring.  This works, but it weakens the spring and it will eventually break.  The second way is to make a replacement leaf spring, out of thinner material.  Things I've seen advocated are hacksaw blades, other thin saw blades, the thin coiled flat spring from inside a tape measure, and clock winding springs.  The third way, and in my opinion, the best way is to use a replacement wire spring.  Full disclosure, I make these springs, and have made well over 5000 of them over the past 20+ years.  The wire spring will reduce the pull to generally in the 3 pound range if the sear geometries are correct.

The thing that you absolutely do NOT want to do is hone the sear geometry different.  That causes the mainspring to "help" pull the trigger, and causes an unsafe condition that can lead to accidental discharges.  The other counterproductive thing to do is to thin or lighten the main spring to try to reduce the rubbing friction of the sear surfaces.  This can give ignition problems, and most importantly can contribute to the action blowing open when the rifle is fired.  Don't do it.  If you want to lower friction, have someone with the correct skills harden and then polish the trigger nose and sear notch; hardened steel has lower friction.  Do not try to polish the sear as is.  It is originally case hardened, and honing it cuts through the case, and the softer steels drag more.  The result is a harder trigger pull, and increased creep.

I believe the picture Schuetzendave posted is one of my trigger springs.  Those look pretty simple, but there a couple of nuances to them that cause them to work better, and give less apparent creep than a homemade spring.  Not a lot less, but nonetheless, noticeable.   

Hope this clears up things a bit.

This is a trigger spring I made myself (not your trigger spring SSDave) with Precision Brand 25 gauge .059 " or 1.499 mm spring tempered music wire.

I have also made them a bit lighter with Precision Brand 20 gauge .045 " or 1.143 mm spring tempered music wire.

I have been making these for friends for twenty years now.

Those wire springs are the best thing since slice bread!!  Tom

I have had several rollers over the years that have a damaged trigger and or damaged hammer. In these I build up the surface with weld using piano wire as the filler with a tig torch. This makes for a hard surface but it can be cut with carbide in the mill. With a milling machine the parts can be cut to precise dimensions for a good trigger pull.