With the search term "BenQ TH683 lamp", you will find plenty of hits on the net.Numerous suppliers have "specialised" in lamp replacement for beamers.
The original lamp with base is available for 200 to 240 francs. But there are already alternatives for about half this price or a little more. Very savvy people could also just buy the bulb without the base, but I wouldn't dare do that, for example. I would change the entire lamp segment, which is probably what is recommended. I am very satisfied with the beamer and have not had to change the lamp so far. But it is not used very often in our house.
I hope this answer helps.
Best regards and good luck