Hi,
I have an ENDA 16 inches folding bicycle with six gears. I got it form my friend in Canada in March ,2014. But the problem I find these days that the retainer type bearings in headset (16 nos 5/32 balls ) have given out and needs replacement . I have seen and checked the other bearings fitted in front hub,crank axle and rear hub are all retainer type. Now I have two queries to place here :
1) There was a slight front-backward play in the headset . I made a an enquiry to ENDA and they said this nominal play is normal for user safety as the wheels are only 16 inches. But the play has increased considerably and I doubt that the Retainer Ball bearings have worn out.
2) In India, it is very difficult to find retainer type bearings and please tell me whether I can go for settling with plain ball bearings in all the places ?

There shouldn't be any play in a headset. Just adjust it correctly. As for the retainer - you can run plain ball bearings in there you will just need more of them as the retainer takes up more room and keeps the bearings equally spaced. It's also just a pain in the butt to put it all together with loose ball bearings. Use a lot of grease to help keep all the bearings in place while re-assembling.

You can use loose ball bearings, no problem. Use grease to keep them in place when assembling. There should be a gap with all balls in place, don't try to fill completely the cup.

I agree, no play in the headset.
UNLESS something is worn, bent, or out of alignment, in which case it should be fixed.
You may worry about friction in the hubs, but not in the headset as long as it doesn't stick.

The local co-op here has a mound of excess headset parts to dig through, or a bike shop should have the the proper bearings.

Well, I am going to fix the issue with loose 5/32 in. balls ( 25 nos probably for bottom and top each) and let me see the results !