@Model24 to each his own, you did get me to thinking about how cool it would be to use one in a vintage gun shooting match.
If you're interested in getting extra mags, Numrich has the best prices I've been able to find; they're less than half the price of any on eBay, and newly manufactured
https://www.gunpartscorp.com/products/1478370A

@Yukon254 if you're still paying attention to this thread and still debating buying this rifle, I'd say go for it.
Their asking price is fairly standard, and while that may seem high for a decades-old gun, the 99 has a reputation of excellence for a reason:
120 years on it's still being listed as one of the best deer rifles of all time and only stopped being produced because it became too costly to manufacture.