I bought a Miracle Belt for my son and it really helped. Its weighted and they wear it for twenty minutes at a time. Worth the money. I also see weighted vests online. The trick is to not allow them to wear it all the time or it looses its effect on the brain because the body becomes adjusted to the extra weight and no longer feels grounded.

Buy an inexpensive (can't remember the actual name) vest, the bubble looking ones...they look like a coat with no sleeves. Get some hem tape. Cut the seams open and remove the stuffing. Fill zip lock bags with playground sand and wrap duct tape on it...covering entire baggie. Insert them into the seams. Use the hem tape (buy the iron type or peel & stick velcro to reclose the seams.

Hello there is a swimming vest that allows you to slide Styrofoam pads to allow buoyancy very inexpensive &17.99 and instead of using the foam you can use weights or fill zip locks with beans or something heavy Bought mine at toys r us Green Frog Swim Trainer Swim Vest - Small/Medium
http://www.toysrus.com/product/index.jsp?productId(Phone number can only be seen by the question and answer creators)&prodFindSrc=prodCrossSell

have they tried a pressure vest instead? my 5yr old alternates the 2 every 30 minutes during his day, but really I would rather him be allowed to wear the pressure vest for as long as needed instead-more consistent results & the effects do last anywhere from 5-7 hours (we took ours to disney with us is how I discovered this-it takes about 15 minutes to take effect if we put it on him during a meltdown but the "off switch" gets switched & everything improves dramatically) the weighted seems soo heavy/awkward to me- but that is the alternative-its rather like a bear hug wearing the pressure vest & is comfy