XBee S2C Modules are available on AliExpress and can be hooked up to Arduino/Raspberries. They are surprisingly expensive though (didn’t see one for less than 22€). But with this I guess you would have to figure out a lot of the details…
There’s also a professional ZigBee module available from a german company (https://www.dresden-elektronik.de/funktechnik/solutions/wireless-light-control/raspbee/) for ~35€.
They also sell the “ConBee”, which is a ZigBee USB gateway for around 50€.
Both work with HUE, but don’t have Tradfri support yet.
Also they are both more expensive than an official Tradfri gateway… so I think the way to go for now is to speak to the gateway instead of directly to the bulbs.