There are basically three options for Mexican Insurance. We've done them all but have to recommend #1. We'll stop at the last exit before the border for anyone who wants to do #2. That sounds gross, sorry.
Get your Mex Insurance online before so you don't have to hassle with it. There are lots of places in the googles to get this stuff, but our favorite for the last several years is Baja Bound
2. Stop in San Ysidro and get it at the last minute.
3. Just run without it and hope for the best.
I've probably spent a fortune on this stuff over the years and never once used it, but still get it just in case. Yes, in younger, broker and dumber years I've skipped it and made it out alive but I'm not recommending that. If you want Mexican Insurance you'll need proof of US insurance, current registration and a driver's license. The cost depends on your bike and how much coverage you get but is generally in the $20 per day range.