Polaris - by American Water Heater: www.americanwaterheater.com - as a homeowner with a Polaris, you may contact their technical support department directly.  We’ve found them to be very helpful and effective when it comes to support issues.  They can supply you with Polaris replacement parts from the factory...

For all other heat sources (TriangleTube, ElectroBoiler, Trinity, and Bock) their technical support staff will want to talk to a qualified technician.  Additionally, they are unlikely to supply parts directly - which is why it’s important to 1) identify a local tech and 2) identify a parts supplier.

While these manufacturers may not sell you parts directly, they may be able to direct you to a nearby supplier who can...

Online sellers of these boilers are possible sources for parts, it is unlikely that they will offer much in the way of technical support.

Polaris Water Heater

TriangleTube - Prestige Boiler

ElectroBoiler