I think the Aber barrel may be out of production. It was 35-L-120, but its no longer listed, you could try e-bay.
I haven't found anything about an RB barrel, but did find these on E-bay, but I have never heard of this company.
www.ebay.com/.../251243703944
And this from Mk 1 Design, haven't heard of these either.
www.ebay.ca/.../251241366334
I am a Norfolk man and i glory in being so
On the bench: Airfix 1/72nd Harrier GR.3/Fujimi 1/72nd Ju 87D-3