If you're REALLY careful, you might be able to drill out some CPVC to accept PETG, so you can stick with CPVC couplers, but use a looser barrel material.
Lee's rigid PVC aquarium tubing comes in 9/16"OD:0.50-51" ID, which is great for barrels, and 5/8"OD:9/16"ID, which both nests over the barrel size and couples in to 1/2"CPVC couplers. I use it to make CPVC RSCB clips too. Another option for this is 1/2" copper tubing.
NH has an old saying that you should adapt your barrels to your foam supply, not the other way around. It takes *far* less effort to tailor barrels to your existing foam vs tailoring foam to your existing barrels.
Which I've been telling you guys is flat wrong
for five years. The absolute easiest and most consistent method that I have found for ensuring good dart fit is this, along with straightening the foam all in one procedure:
1) Decide what you want to use for barrels. In my case, I chose normal 1/2" CPVC (for springers) because of cost and compatibility, but there are numerous reasons.
2) Get your too-large-foam, the longest possible sections of a tube slightly tighter than your chosen barrels, and a piece of string a little longer than the tube. I use Flowguard Gold CPVC for these tubes.
3) Tie a weight to the string on one end and a slip knot on the other.
4) Tighten the knot around the end of your foam.
5) Drop the weight down the tube and pull the foam through until the knot pops out.
6) Cut off the trailing end of the foam and remove the knot from the leading.
7) Repeat for the all tubes.
8) Leave for a week or two.
9) Pull the foam out a little at a time and cut blanks immediately.
I will to this fifty feet at a time in five ten foot sections of pipe, and just leave them in the rafters of my garage. When I am making darts, I just pull out a section and cut it. The sections are only a few dollars each and can be used for plumbing if I need it.