I really prefer the OEM rubber boot from BMW. Avoid MTC, they're cheap but only last like a year.
ECS tuning is close to me, had the OEM boot for about $60 IIRC. Pelican Parts, AutoHausAZ and BluntTech are all fine vendors too.
Samco used to make an E30 M42 intake, English company AFAIK, but I'm thinking those are all long out of production.