I love the concept of vac-u-lock (interchangeable dildo mounts), but the inventors (Doc Johnson) seem like they had one good idea and gave up. I'm compiling my own research here for myself and others.

$ - under 50 USD

$$ - 50 to 100 USD

$$$ - over 100 USD

Doc Johnson - Wide selection of meh quality. Dildos tend to be 'celebrity' cocks in PVC and the silicone ones I've used are ultra hard and unyeilding. The mounts and accessories are plastic, and just okay. Suction cups aren't great. If you want a cheap smaller dildo the 'Vac-u-lock Smooth' silicone one is reasonable. Best accessory is the 'Knuckle up' handle which despite looking aggressive is very comfortable (IMHO). $-$$

Tantus - Moderate selection of 'suction cup compatible' silicone toys (realistic & abstract). Tantus is good quality and they have soft, hard, and dual density options. Have not tried silicone suction cup. $$$

Squarepeg - Moderate selection of 'SquarePegHole' toys in soft silicone (realistic & abstract). Squarepeg gets rave reviews but their toys tend to be large. No accessories. $$$

Hankey's Toys - Wide selection of realistic of multi-sized silcione toys, but tend to be quite large. Toys on the 'smaller' size like Topher Michel or Cockmaker are almost can sized. Two solid looking suction cups available, one super-sized for big toys. $$$

John Thomas Toys - Wild selection of multi-sized realistic and fantasy toys in silicone. Also tend large but more smaller versions than Hankey's. Unfortunately their website is a pain to navigate. Aside from sizes they also have different silicone firmness. $$-$$$

BadDragon Toys - Limited selection of 'DragonMount' fantasy toys in silicone. I say limited because they 10 DragonMount designs at time of writing but only 1 available for sale. That said, different sizes and hardnesses available. $$$

Fleshlight - Moderate selection of silicone celebrity dildos, plus a couple of abstracts (HerSpot). Unsure of quality. Not obviously named, so have to dig around, some say they have 'silicone plug'. Also some very sensible looking and well-priced 'dildo handles'. May try myself as a substitute for Doc Johnson ones. $-$$

Other

ShevchenkoDesign - Make one of the better dildo handles I've seen in aluminium. Also decent fucking machine accessories and parts. Featured dildos don't get good reviews. $$

Blush Novelties - Also make a solid aluminum handle, but only bundled with a tacky overpriced dildo (WHY??). Called the 'Lock On Pleasure Tools Karbonite Aluminum Handle'. Their 'Lock On' range of dildos is very limited and in PVC. They also have a spin-off 'The Realm' dildos in silicone, and some mildly hilarious plastic sword handles.

HUNG System - Wide range of plastic handle accessories. Some large dildos and buttplugs, sadly all in PVC. They have a large base system for sitting on which may be useful. $-$$$

Etsy - Wild west of accessories for your harness, theragun and anything else. Some dildos but lean fantasy. I personally find Etsy a lot of work to sort out quality from crap. Also leans expensive. $$-$$$

MOD Wands - simple silicone wands at 1.5 diameter size. Actually nice to see something smaller. $$

Boneyard Toys - sells dildo, suction cup, handle sets. The handle looks dope if you have nothing else. Review here: https://phallophilereviews.com/dildo-with-handle-boneyard-silicone-kit/ Unfortunately the reviewer says the handle fits less than perfectly than other vac-u-lock handles.

Make your own?? - Hanky's Toys have a drill bit for making vac-u-lock sized holes. No knowledge on how well this works.

TL:DR

Best smaller vac-u-lock dildos: Tantus

Best large vac-u-locks: SquarePeg, Hankey's Toys, John Thomas Toys

Best cheaper handles / accessories $: Fleshlight, Doc Johnson

Best fancy handles / accessories $$: ShevchenkoDesign

Best suction cups: Hankey's Toys (?)