WoodSmith: SWAGED lead bullets,not cast,like by Hornady and Speer,will do the job,if you can't find a true slug. These are made in the 2 sizes you want-plus they are very accurate,unless you want't to "crank them up" into the 1000 fps range! Then it will be, "time to get the lead out"!!
I keep a few pure lead muzzle loader balls handy for checking bore size. Pick one oversize for the bore you wish to check, start it with a plastic mallet, and drive it through with a wooden dowel. A light swab with an oiled patch prior to starting the lead ball seems to ease the process.
Make certain the bore is clean and lead free before slugging.
Woodsmith, you can get a bunch of lead over to any 'handloader' who casts their own bullets.
Trouble is most all the lead you come across has SOME 'other' alloys in them, NOT pure, soft lead. The tin, antimony, etc is needed to make a "proper" bullet.
Have NO clue why you'd want to put a 'soft' lead bullet down a "fine firearms" bore.
If you just want to "slug" the bore (make a measurement) then get some 'Cerrosafe" from Brownells and follow the directions.
Or do as said above, just pound a round ball down the bore! (please don't use a 'steel' rod though)
From you "subject title" sounds like that's all you wanted to do.