Where can I order a quality CV boot kit for both sides, boots, grease, gaskets, etc...

Pmb, pelican, probably amazon and eBay.

Your Kit will be for a VW. To the best of my knowledge NO 4 bolt hole boots are being made. Everything is 6 hole.

If you need some I have some 6 hole that have the two holes sealed with epoxy and a aluminum plate. I upgraded to the the indestructible ones (almost) used for V8 conversions.

can you replace the boot on the flange? The boot sells for $3.

All the boots I have got do not want to stretch over flange. They pop off before you can get a clamp on them.

Problem is the original version of the flange which interfaces with the boot has a curved surface, the original clamps had a matching curve they are NLA.
Owners have tried everything from tie wraps to the straight edged clamps which have the correct diameter but lacking the curved edge.

uh... I replaced all of my boots and reused the original flanges. Getting the boots over the lip of the flange takes a bit of strength but once it's stretched over it has the added advantage of retaining the boot a bit. Clamp added of course, wouldn't want to rely solely on the stretch.

I used a bike tire lever (shown in blue in the picture) to help leverage the boot on without risking puncturing the boot.

I believe mine were Rein boots, purchased from FCP Euro. Price currently is a bit over $2 each. FCP also has a lifetime warranty on all parts they sell and you don't need to be a professional shop to take advantage of it.

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I use a heat gun to warm the boot a bit and it makes it much easier!