Starting issue/ air leak through throttle shaft

[NH603]

I'm having an issue with my 1999 gz 250. The engine will turn over when I hit the started but does often does not start. When it does start, it seems to do so most easily when it is warm out.

The starting issue is relatively recent, but for as long as I have owned the bike (almost 2 years), I can't ride it until its been warming up for a few minutes (otherwise the engine immediately stalls out).

I have cleaned out the carb and jets many times, changed out the intake boot and o-ring, and it hasn't helped.

I have identified that there is an air leak through the throttle shaft (from spraying with carb cleaner while idling and hearing the engine rev up). I am hoping that this is the cause of both of my problems. The issue is that I can't find a rebuild kit for it online. I think I just need to change the seals out.

Any advice?