Edit: Is this message visible? I keep getting error messages.

tl;dr: Get the O'Connell TKO (though I haven't used it, conceptually it is a logical improvement to the current baller head injector/diffuser lineup across the board, the way it is built the plant material is closer to the heating element so you can get similar effects at lower temperatures, which means less harsh vapour).

I personally went the aliexpress route, with some DIY bits and bobs, mainly because I am in Europe and am on a budget. My favourite head I own is the Tiodw Mini, which is a titanium enclosure. I prefer a lot of airflow so I use the larger 4mm balls. The store I buy from is b2cvape, their newest head is the Tiodw Mini 2.0 (which is a hybrid injector and diffuser baller head). It will probably be my next purchase. But with this route you also need to buy 3mm or 4mm rubies (made from corundum), a 20mm coil, a PID and a bowl (the new head is both an injector and a diffuser so getting 2 types of bowl is a good option), a handle (not 100% necessary) and some sort of bong. Excluding the bong you can get all of this from that store for around 150USD.

However, if you're in the US or have no budgetary issues, you should look into Cannabis Hardware products. Their entry level B0 is a tried and true daily driver for a lot of heavy duty users. They also released a similar double sided (diffuser and injector) head attachment recently called the Tomcat, and 2 new more budget oriented heads the F16 and F22 (one is a diffuser and one an injector, you use the Tomcat to stick these together).

Another great option is the Malaysia based Qaromashop, they have great glass and ceramic baller head options. Their Qaroma heads are glass (its weaker but has better flavour), Ceroma are ceramic (really great budget option, the aliexpress store also has a ceramic option which was my first ball vape and is also great for taste). Their Taroma Lite Plus seems like the good budget option they offer.

There is a company called gophase3 which sells smaller baller heads which fit 14mm bowls, they look very interesting and use zirconia for some of their baller heads which I have heard tastes great and hits as hard as larger baller heads which contain more balls.

The O'Connell TKO seems to be a favourite among serious vaporists (lol), their design moves the plant material and the heating element closer to each other so you can have great effects at lower temps. I think this would be the one I would get if money/delivery wasn't an issue.