This looks like it should fit the bill, though I can't vouch for it since I've never used it, however, eBay does have some good reviews on it.
DAC Optical Toslink SPDIF Coaxial Digital to Analog RCA Audio Converter 3.5mm 733180335982 | eBay
If you search eBay for the search terms minus quotes, "optical to spdif." you'll find all kinds of solutions.
Here's a tip. I like shopping mostly on eBay myself because I don't have Amazon Prime since I don't buy enough from Amazon to merit it. But I often find items on eBay cheaper by comparison and often times with free shipping with no Prime required. Then sometimes, and for some strange reason the seller on eBay ships your item via Amazon! LOL I imagine they may put an end to that, but at present it doesn't bother me so long as I got it cheap, with free shipping and it actually came in three days or so.
Something else I'll do is compare the same product/s' reviews at Amazon and eBay and use this site to gauge the reviews for quality. Note, this website doesn't give a score on the product its self, but the overall quality of the reviews on whether there's deception involved.
www.fakespot.com
Now for some useless information in case you were wondering. LOL SPDIF stands for
Sony
Philips
Digital
Interface.