I recently bought a magnificent (expensive) Beretta 692 that came with exchangeable chokes, but not enough. Why they can sell a gun like that without supplying the full range of chokes, I do not know.
For my clay-shooting (and hunting) I would like a light-modified and a full choke. Beretta's website supplies them at quite a price but the confusing part is the myriad of other chokes available from other sites.
Question: how much do after-market choke tubes vary in quality?
Advice will be appreciated.

I don't know what you mean by quality. If it's pattern it's a moot point.
Regardless of the what the choke is you must always pattern the load .Lead, steel, bismuth shot ? Small shot , large shot ?
My Benelli with steel BB shot ,MOD choke , patterns EXTRA, EXTRA Full !!
Get one of the better brands and pattern .

Thanks. Living in South Africa it is quite a commitment to buy an item from overseas and to return it is impractical, hence the aim to research as much as possible.

I can not talk to varying quality,, but I have had very good results with Carlson choke tubes in several gauges.

Have used Briley for years and have a very large following of shotgun shooters of all types. Just a suggestion.

I can't tell any difference in quality of internal, or flush, chokes and I have/or have used chokes from eight different companies. But my wife has a 28 gauge that came with a six choke set of external, or extended, chokes made by Seminole and they are, without a doubt, the best made chokes I have encountered. Unfortunately, the business seems to have disappeared.

Thanks for the tips so far.
There is a bewildering array of chokes available for a wide range of guns and spanners, so getting the correct one will be something.
I will phone the shop where I bought the gun and ask them to ensure that I get the exact fit.

I have a set of Teague after-market chokes for my Browning 525. They work like a charm. Teague makes chokes for just about everything that can shoot a shotgun shell, and then some. Ramrod in Jo'burg used to be their SA agents.

Karoo:

Unfortunately my Eastern Cape days are probably over, but if you finally decide on the size and type of tubes you want, a request in here MIGHT get you a set delivered by a Cape hunter for US costs. I've bought quite a few used tubes on ebay for US$5-10 each including Berettas. These days many people don't use Full chokes for hunting and if you got a handful of them, you could have them bored out to your specs. Something to consider.

Thanks very much Gato. The dealer is looking into it and if I don't come right there I may well take you up on the offer.

Have had good results with Carlsons.

With Beretta , make sure you are ordering correct style . My Silver Pigeon O/U uses Optima Plus choke tubes, while my Beretta 391 semi-auto uses Optima choke tubes. They will not inter-change.

I prefer the extended tubes, easy to change and size is marked on exterior.