You have two options for building multi time....become an MEI or buy it. I had co-workers who bought blocks of multi time and hopped around the country. I chose to get my MEI and build it that way. Right now, I'm one of only 2 MEIs in our flight school so I get a reasonable amount of multi time each semester. Flight instructor pay is not the best, but it's certainly not terrible. Right now every single flight school in the country is looking for CFIs. We have 3 leaving in the next 2 weeks, and lost 3 additional CFIs in the previous month. Bigger flight schools will pay more than your average Part 61 FBO will.