Hello zerograv,
Can't speak for all rental outfits, but in Southern Ontario I am unaware of anyone raising their rates during the summer months. It might be different elsewhere in Canada but I would be surprised.
It could be that you were quoted a higher rate than that specified on a website simply because the website was not kept up to date. I know that most places raised their rates last Fall (allegedly because insurance went up post-Sept. 11th).
Regarding availability, you should not experience too much difficulty finding a 150 or 152 to rent for time building. Generally speaking the larger outfits (e.g. Buttonville, Brampton) have more demand on their trainers than the small operations (e.g. Guelph, Oshawa). That's probably true across the country.
If I were you, I would surf the web and find two or three schools that look good, and then post another enquiry here asking if anyone has any experience of those specific schools. Here is a link that should get you started:
www.avroland.ca/al-cdn-aviation-schools.html
Hope that helps,
MLS-12D
P.S. $100 per hour for a C150/152 seems high to me. Prices vary from place to place, but I would currently expect to pay somewhere between $75-$90 (wet). See e.g.
www.durhamflightcentre.com/