I too have been looking for transformers. What I've learned is:
* For solid state, the options are extremely limited in the US. I've found nothing but toroids. R-core, C-core and EI core are available but principally for tube amps. I've found none of these types specified for the power supply in a solid state amp.
* For toroids, Plitron has the best reputation for quality and low noise. The drawback is price, upwards of $200+. Antek is a Chinese supplier favored by many DIY'ers for decent quality and low price. Hammond and Avel Lindberg are a step up in quality but more expensive.
* Example: Plitron low noise 500VA Is C$304.52 available in 10-15 business days
Plitron standard toroid same specs is C$136.67 available in 10 business days
Anatek with similar specs is: US$55.00 available now
Avel with similar specs is: US$86.16 available now
Hammond with similar specs is: US$120.31 available now
Hope this helps.
Jim