There is an early intervention catalog called "Beyond Play" that has great products that may give you some ideas. The website is beyondplay.com They have a book called, "Start to Finish: Developmentally Sequenced Fine Motor Activities for Preschool Children" that may be up your alley. My son's OT comes this morning, so I'll ask her for book and activity ideas.
Working with playdough and clay are EXCELLENT (and fun) for building fine motor skills, specifically strengthening those little muscles. Games with spinners, or something like "Hi-Ho Cherry-O" with smaller pieces to grasp may be helpful. Water play with droppers (like the ones that come with infant tylenol), pouring water or rice from one container to another may be fun. Teaching him to help you fold the laundry - like wash cloths or dishtowels may be fun and helpful. Songs with finger plays like "Itsy-bitsy spider" and "Where is Thumbkin?" are good for fine motor too. Peg boards are a really good tool for fine motor development. Check you phone book to see if you have a toy lending library in your area. They may have some of these things that you could check out for a couple of weeks at a time. The cool thing about that is the toys are special since you only have them for a little while.
Oh, I found a Sesame Street Board book called, "Snap, Button, Zip" a while ago. I got it as part of a set through the Pampers Points program, but you may be able to find it on amazon.com or somewhere like that. School supply catalogs (I love Discount School Supply) may have something you would like in that category, but it may be more money than you want to spend.
Good luck! With a caring, attentive parent on his side, your son will be ready when he goes to Kindergarten.