One of the biggest sites is chrono24.com, which is essentially ebay for watches. The majority of sellers will use the site's integrated escrow system so there's little risk of somebody failing to ship.

To get started just enter "soviet vintage" into the search-box. Once you have some results click on "Filter" and you get the option to set "Location: European Union", "Max Price: €100", etc.

There are a lot of regional variations, so I use chrono24.fi, for example. But the content is the same - I think it's just the default location and currency that changes based on TLD.

The site has been around for many years and is well-regarded, but even so I don't think I've ever paid more than €1000 for a watch there. Just in case.

Edit: Main thing to pay attention to are the dimensions of the watch. Watches from the 40s-80s tended to be smaller. So you'll find diameters of 34mm, 36mm, etc. Most people prefer larger watches these days. For me 36mm-42mm is fine, but that's because I have thin wrists. But I know looking at photos can be misleading in terms of diameter/dimensions so read the details.

Edit #2: This is what I'm wearing today, which cost me around €70 - https://tinyurl.com/dd7bnthx