well.. first measure for the size.. you will need the inside measurement.. and the length.. also how thick it is..
around here.. ACE hardware carries u bolts.. also.. you could try a trailer place.. or a farm equipment joint.. our True Value does also.. even try some of the auto parts places..
any decent spring shop would have them. any place that rearches springs, probably has a large selection that they will sell cheap. thats where i,ve always gotten mine. i take one in and match it up with what they have. start in the yellow pages.