... you'll definitively like the ones made by RB Models. AFAIK For a while, they made barrels for Armorscale and now they're selling them under their own name (for a lot less, sans instructions). You can get them (among others) from Track48 or Fightigh48th.
No relation to the manufacturer, just a happy customer (etc. etc.).
"Car's grip is greatly reduced when wheels are not touching the ground" - Steve Matchett