Like most things in life, they sometimes turn up on eBay. Just remember: they were originally available only in Europe, so you'll have to pay import shipping. Even in Europe, G7400s aren't common. Don't expect to find one right away, but do expect some competition while bidding.
Click here to search for "G7400" on eBay (this link searches internationally).
Another alternative to consider is to browse the forums on some of the major classic gaming sites, like Videopac.nl or Atari Age. Some of their members live in Europe, and may have G7400 consoles for sale or trade from time to time.
There are two basic varieties of G7400 consoles out there that you're likely to find at anything approaching a reasonable price. The first model has an 8-pin SCART output. With the proper cable, you can use the SCART port to get RGB video out of your G7400. So, if you own a TV or monitor capable of displaying RGB (or one that has an SCART input), the SCART model is what you want. Sometimes this console model is known as the Philips G7401.
You can recognize this model of G7400 by looking at the back of it. If there are three circular (DIN) ports present, it's an SCART model. The other two ports are for the joysticks. Note: In France, where the SCART models were distributed, SCART is known as Péritel. If you're browsing an auction listing from France, expect to see "Péritel" rather than "SCART."
SCART model G7400s were originally packed with a DIN to SCART cable. If your TV has an SCART input, you'll want this cable. If your TV lacks SCART capability, it won't be as important.
Buying an SCART model will make it much easier to get your G7400 up and running. Unfortunately, the SCART model is rarer – and may be more expensive – than models without an SCART port. Additionally, it won't be of much use to you if you don't own a TV with SCART or RGB inputs. If you don't, keep reading.
Don't panic – you can still use a G7400 even without an SCART port. However, you'll need to physically modify the console to get it to work. Consult Step 2 for instructions about how to modify a G7400.
An obvious option is to buy one. A reasonably low-cost RGB monitor is the Commodore 1084, a favorite of classic gamers everywhere. This monitor should be compatible with the G7400.
Click here to search for "Commodore 1084" on eBay.
If you'd rather not buy a new monitor, it is possible to modify the console to output composite video. If you decide to pursue this option, then you should buy a G7400 without an SCART port (which should be easier to find anyway). Consult Step 2 to learn how to modify your G7400.