Garry, I commend you on wanting to keep spare parts on hand such as brushrolls for your vacuums. Always a good idea, especially for the ones you frequently use in your collection.
However, for a Singer/Kenmore Twin Fan, because of the age of your vacuum, jury rigging may be your best option here. Since you can remove the brush strips from the brushroll and replace bearings, what I would suggest doing is finding brush strips from another brand that the strips are similar enough they will fit and slide in easily, or will work with slight modification. One that should work, I would think, is the brush strips used on the Kenmore Powermate canister powerheads. Or brush strips from the Riccar/Simplicity full size canisters with the 3 row metal brushroll in the powerhead might work too.
Rob