Drip irrigation fitting: Where can I find drip Irrigation fittings for 1/2" tubing to 1/4" line that fits .620" to .710"?

[FromCali]

My main tubing fits Power-Loc 60-series fittings well. I got them from a local store and the description reads "60 Series fittings fit .620" to .710" with wall thickness from .045" to .055".

I am using poly tubing (1/4" line) for some areas. My problem is that when I put a 1/4" barbed connector in 1/2" main tubing, it licks a bit. I bought these to solve that problem. Using these solves my licking issue. But these are smaller than the size of my 1/2" tubing. So I have to squeeze my main tube a lot. The local shop doesn't carry these. When I tried searching online, I couldn't find large enough (.620" to .710".). Any idea where can I find these?

 

[Oliver Buckle]

I have used copper pipe in tank connectors joined to plastic mains water pipe, not a perfect fit but I got round it by coating the copper pipe with gutter sealant and putting a hose clip around the plastic pipe. Not elegant, but it works well.