I got a new 100 amp last year. It made it to the east coast and back and I worked it hard. I think it was from Amazon. I've heard of them also being available through NAPA.
I'm planning to install solar panels this year to charge/maintain the house batteries while traveling instead of pulling power from the charging system which fights to maintain main battery voltage when driving the electric cooling fans I installed. Next step would be more/higher capacity alternator(s). I'd rather not do that.