I was looking to buy beach wheels. I called Murray's and the advice I got was not to get plastic wheels if I would be using them on sand - much harder to pull the boat. At around $500, I could not do it.
Being patient, I was able to make a set for under $60. They weigh in at around 80 pounds though.
I scrounged the following for free -
Rusted out catamaran trailer, no longer usable (Craigslist is great!).
Old ATV wheels/tires that did not hold air
1) Cut the trailer up with a saws-all, saved the axle and cross members that originally supported the catamaran, and couple of 3 foot pieces from the rest of the trailer.
2) I cut 16 inches out of the axle, welded it back together.
3) Put two new inner-tubes in the tires (ouch - cost me $35.00) Bought some tire dressing at the 99 cent store to clean them up.
4) Painted the rims white, the rest black (ouch again, $15 in paint)
5) I got lucky and the axle bolt pattern fit just inside the rim opening - used big washers to "pinch" the rim between the hub and lug nuts.
It works great, very sturdy.

[/img]