I use latex tubing. It works SO well! Plus it's soft and flexible, not hard and stiff like those stupid red tubes. Cut off the length that you like, from the roll. Wet the end with rubbing alcohol. I do injections, so I have a spray bottle filled with rubbing alcohol. Now, take the END of the latex tube, and push it back INSIDE of itself, for a distance of perhaps 3/4 of an inch.
Now, cut 1 or 2 small eyes on the side, at the end. Don't make the eyes big, or the end won't stay straight as you try to insert it. You want the tube to still be stiff enough not to kink. Use the handle of a spoon as a tool to push the tube inside of itself. The size that I use is either 3/8 INSIDE diameter or 1/2. The WALL thickness is either 1/8 or 3/32. A 1/2 tube is actually 3/4 AROUND, counting the wall thickness. Pretty big! Bigger than a cigar!
For new girls, a 3/8 inside diameter works best. I can thread it right in with little or no serious discomfort. I put a speculum in her ass, so that her rectum is open and not squeezing the tube. Then, I just slide the thing in. If it hangs up on a muscle ring, I back it out an inch and work it past the obstruction.
Plus, I have a micro vibrator in the tip of the tube, so I can just turn that on for a second, and then feel her abdomen. You can easily feel the EXACT location of the tip, and simply massage it past the obstruction. I've gotten so good at it, that I can do a lot of the tube with the girl in knee chest. The easiest position is when she's laying on her back.
I'm telling you, forget those stupid red tubes, and start buying some cheap latex. Kent latex is the brand that my surgical store stocks. I'm sure that there's many others too!