If the former, get the sizes and look on eBay (plenty of options), if the latter you may struggle because Ryobi in their infinite wisdom made very few sizes and their bushing generally don't fit anyone else's tools (at least not based on the R-500 and RE-601 I once had). Best bet may be to buy or make-up a new sub-base which can carry a more generally available guide bushing, such as the Trend Unibase (which works with Trend/Elu MOF96/deWalt/UJK guide bushings - widely available). Leigh also do a guide bush adaptor (#703) for the R-600/RE-601 routers which will accommodate the Porter-Cable style threaded guide bush (available from Axminster, etc)
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