With 3/8" OD tubing, you can grab some 3/8 reinforced clear vinyl hose at Lowes or Home Depot, push it on and clamp it. Then you can use hose to barb fittings which unfortunately are kinda pricy if you can't find a plastic version in the garden hose section.
Another trick is to use 5/8" garden hose but first slide a short ring of 3/8" thinwall hose onto the end of the copper as a reducer. Then push the 5/8" on top and clamp it. The benefit is you definitely know you can use the cheap $1.50 male/female plastic hose connectors.
So many people put the hose connections directly on the copper chiller but then you have to use two garden hoses to move the water in and out. If you're chilling pretty close to your water source, you might as well just have a few feet of hose connected directly to the chiller. This is especially true on the output side. I only have 5 feet of output hose and I let it dangle into a 22 gallon rubbermaid tub. I use this for cleanup after the brew.