Both of those questions are best answered in game to be honest.
Most nations have at least someone with each realm mastered, so it will be a matter of going around and asking your groups. The egregores may be able to help point you in the right direction at least.
Otherwise the archmages should know, they will be very busy, but may have assistants who could help.
And then you will need to either persuade someone with a lot of spring power that you are worth their time - they are likely to be busy, or you will need to persuade a coven that you are worth one of their 2 coven slots for that day.
I would suspect the amount of money/resources etc that you are willing to spend on it will greatly affect how easy it is to find someone to do it.
And all of the above depends entirely on which characters are at the event, and who you know. There is very little that can be generalised.