of the original 1903s there were only 2 manufacturers, springfield and rock island, remington started making 1903A1s with the equipment from rock island and eventually started making the A3s along with smith corona.
if your gramps just had the plain jane 1903 then that makes it pretty easy. the majority of those available are springfields, there weren't a whole lot of the rock islands made in comparison. neither are really more sought after than the other but I tend to gravitate to springfield and remington with old US war horses but that's purely brand preference.
one thing to look out for is low serial numbers, early metalurgy practices were primitive even by turn of the century standards. stay away from any springfields with a serial number lower than 800,000 and any rock island lower than 285,000. that's about all there is to it, as for where there are no suppliers for them any more, all sales are dependent on someone selling theirs. gun shows are the best place to find them. 500-700 dollars is about average depending on condition but I suggest you look at a lot of pictures so you can identify when something has been modified as that can greatly decrease the monetary value.
ignore my complete lack of capitalization. I still have no problem correcting your grammar.
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