You can get at least a 4 ton gantry thru amazon
Vestil AHS-8-15-12 Steel Adjustable Height Gantry Crane, 8000 lbs Capacity, 15' Length x 10" Height Beam, 91"-145" Usable Height: Amazon.com: Industrial & Scientific
www.amazon.com
Vestil AHS-8-15-14 Steel Adjustable Height Gantry Crane, 8000 lbs Capacity, 15' Length x 10" Height Beam, 103"-169" Usable Height: Amazon.com: Industrial & Scientific
www.amazon.com
When I got my first gantry (Haven't built the second or third yet) I figured my OCD level would make it take a LOT longer to build than buy (boy was I wrong) - so I got the HF 1 ton - I ended up doing enough mods due to its sortcoming I'd have been better off just starting from scratch;
Things I considered shortcomings -
1 - Barely 8' between uprights, and NOT adjustable width
2 - Very little bending resistance at the post/beam joint
3 - REALLY crappy "system" for changing beam height (Still not fixed, but I got a plan AND the parts

)
Pic 1- the original beam is 1/2" narrower and 1 inch less vertically than the 6" 12.5 lb beam I replaced it with, + the new beam is 12' long so I have just over 10 feet between posts - plus my mounts are a teeny bit stronger
Pic 2 - Unfortunately the original beam mount plate (welded to the post) is still there, so...
Pic 3 - I added adjustable gussets from 4" channel and 2"x .250" wall square tube - the tubes are far enough apart to accept either a 1 ton electric hoist or a chain fall WITHOUT limiting travel along the I beam nearly as much as a single in-line gusset each end. Also, the gussets I built can be loosened and slid ANYWHERE along the beam if necessary.
Pic 4 - You can see that with the gussets in place I can't use the two LOWEST beam height settings - if this EVER becomes a problem, I'll mod the lower "clamp" by making it wide enough to clamp on the OUTER post tube - otherwise, if it ain't broke...
Pic 5 - One of the gussets ready to mount.
Pic 6 - Ready to go - until I get another shop area with a 48 x 60 slab, it'll live where it is (a 15 x 24 area under the roof I built over the 20' container) (see pic 7)
Not shown - the original beam from the crane is now bolted between the double trusses, perpendicular to the gantry, with its own 880 lb hoist - I've had over 500 lbs on it with ZERO deflection of the roof trusses, I've used that hoist for the front of my zero turn and two separate chain falls for the two rear corners giving me a 3 point support (with 3 of HF's underhoist safety stands of course) - did the same thing with my little 5x8 2000# rated utility trailer when re-doing the wiring.
Yeah, I AM old enough to remember the phrase "Run whut ya brung"

... Steve