Would sitting on an exercise ball work instead of a wobbly chair? I know ADHD kids that benefit from it because they can move/wiggle/bounce. Obviously ADHD and SPD are not the same thing, but maybe that would work. I mention it because you can find exercise balls for pretty cheap.
Maybe the special needs board would know of great resources too.
My son is sensory seeking. We are starting our first year of home schooling, so I don't have any experience, but my plan is to count on a lot of breaks for movement and to encorporate sensory breaks as well. My son loves playing in the sand and thera putty, so if he's getting figity, I will have him take breaks. He can also do school work while on the swing or playing in the sand or with putty. We have a swingset outside but also a swing in the living room.
Three of my kids have sensory issues.
Exercise ball, lap pad and music.
You can make a weighted lap pad with a half a yard of material and a package of beans or rice. This is a little easier to use than the full size blanket during school work.
My oldest does her work with far less problem if I let her listen to the mp3 player while she works.
Do you own the book Out of Sync Child has Fun? I've been using activities in there to break up our more formal schooling activities.
Best of luck.
attached to DH 10/03, DD1 8/06, DD2 12/07, DD3 5/09, DD4 11/12