Your little guy love trains? Mine does. I've recently discovered that AC Moore, the craft store, carries pieces for the Thomas the Tank Engine all wood train set. You can get a great deal on pieces for your train set by using the 50% off a regularly priced item coupons listed on their website (you can also sign up for emails where they will email them to your inbox too).
Forgot your coupons? No problem. Their fliers at the door almost always contain one of the 50% off coupons. By far the cheapest place I've been able to find pieces for this set.
Look out for sales on the the pieces too so you don't have to bring extra bodies with you to buy more than one piece (only 1 coupon can be used per transaction).