It depends on the type of fan, most rvk style fans are ac not ec, ec fans typically are more expensive.
So when you get something like a ram fan with built in speed controller its an ac fan with a speed controller that works on pulse control , for it to have a variable transformer built in it would weigh twice as much and have essentially a house brick size and weight attached. Never seen one on sale.
The ac infinity is a good value EC fan that has lots of positives but one negative is the temp controlled one turns off when temp is lower than desired , which is not practical in a grow room. But the speed controlled version can be turned up and down as needed and wont hum.
As Bukvicak said in the EU they are available from diyleduk.com, he is the only shop to sell them at least in the UK since he imports them from across the pond.
