That's how I usually find out about seminars - I'm on the email list for many rinks and vendors in the US.
Some places pay the PSA to send out an email advertisement for them.
Riedell and Jackson sometimes promote seminars, especially if they are sending a representative.
The bigger seminar team (like Grassroots to Champions) has its own mailing list that you can sign up for.
The USFSA has been trying to start an online list without making any endorsements.
If there's a special seminar or workshop during an ISI event, they promote it on their Facebook page.
www.skatingforums.com has a forum to post "events for skaters" which gets the word out to a lot of skaters and parents for free. I was posting them on the Calendar, but most people said that they don't look at the calendar on the board, so I stopped, at least for the summer.