I have a number of sizes of hollow mandrels or blowing tubes. Sold as an individual mandrel. Based on outside diameter:

• 3mm (thin wall)
• 3.2mm
• 4mm  (thin wall)
• 4.8mm
• 5mm (thin wall)
• 6mm  (thin wall)
• 6.35mm (Australian made)
• 6.35mm (USA made)
• 7.9mm (USA made)
• 8mm  (Australian made)
• 9.5mm (USA made)
• 10mm (Australian made)
• 12.7mm (USA made)

Thin walled hollows are not suitable for leaving in the flame for mixed oxy torches. If you use only the tip or just a hot head these are a good option. 

Astrid Riedel's class and tutorial supply list calls for 3-4mm and 6mm diameter by 12" length Stainless Steel Tubing. The fractional equivalents are 1/8" (3.2mm), 5/32" (4mm) and 1/4" (6.35mm), respectively.

The following tutorial shows how to use a hollow mandrel or blowing pipe for Vessels