Got a couple of donut type things last year from www.boatworld.co.uk
Good service and had quite a range of other bits that emptied my wallet ! - Infaltable fenders / de-salt fluid etc. You'll want to also get a suitable toys tow line plus a 12v inflator and deflator
Used a flat ring cover donut (Buster) for the kids (8 & 6yrs) but a little too bumpy for them on the sea as they got bounced in the air off the deck of the donut too easily. Also got a "Racer 30" which can be more 'sat in' for a gentler ride if required and that was better for them on the sea but it is quite big & heavy to manhandle once wet as it doesn't drain the water out as easily as the donut.
A wakeboard will be as much again for the bindings - you'll need to try some on I think - my first set of bindings struggled with size 10 feet and the velcro wrap over one size fits all set I replaced them with just don't feel as secure though !