Hi S.,
I'm a preschool teacher down here in Portland, so I am unsure of options in your area. But here are some ideas:
As previous posters have suggested, the resale shops (local goodwill, consignment shops or events like JBF) are a great place to check. I have found some great things during garage-sale season, too.
You can make your own playdough, you know. Look around online for a good recipe, or PM me. Fred Meyer (if you have Fred's up there) does sell it by the bag instead of the smaller cups, and I think that's a better value than the little kits. Many kitchen items will work with this activity.
I'd also go online and invest in a good block set. IKEA has one with a cart, but I have found there aren't enough blocks to use later on as they get older. Constructive Playthings or Community Playthings has good sets, and they are more expensive, but in my opinion, an investment item. If you can afford even a smaller set of blocks, great, and the Unit Blocks are wonderful. My group of 3-4 yo's love them, as do my son.
Added here: The IKEA magnetic crane truck with the trailer that offers shapes to attach-- this is worth buying. This toy has gotten lots of attention from both my son and all of my preschooler, boys and girls. The shape pieces fit into the trailer frame, stack, and the kids just love playing with this one.
Okay, here's the deal for Montessori-type toys: they aren't going to be inexpensive. I have purchased puzzles for my school at the discount Montessori websites, and this would be where I would send anyone interested. PLAN toys has some great things, like the Cone puzzle (3 dimensional) which is a huge favorite with kids. Stacking cube blocks (they nest inside each other, and can be stacked into a tower similar to the montessori Pink Tower) can be found online and are often made of thick cardboard or wood. But yes, these quality items are going to cost more. And the Froebel Gifts site is more expensive, but some of these items may be perfect for your son.
Lastly, if your son takes to Montessori toys, great, but please don't worry about his 'being ready' for the toys at school-- sounds like he already is, and the teachers are very well-skilled at introducing new materials to youngsters!
H.