I haven't seen anything specifically for welding, but machinists use precision radius gauges that would probably work. Check out mcmaster.com or mscdirect.com, you can purchase them individually or in sets.

The radius gauges mentioned are fairly inexpensive and easy to get.
All I wanted to add is, whenever I find I need to measure a radius, my gauges are not always handy. In those situations I look around to find round objects and measure their size. Pen caps, coins, washers, anything that works.
If the situation requires more precision than that, then I would probably have my gauges with me.

This is off topic but one situation I found myself in: I needed to inspect welds in another city, so I stopped by the airport TSA office before flying out to make sure my fillet gauges could go in my carry-on bag. They examined them and I was told "no problem". A couple of months later, I had one, 1/8" radius gauges with me, which caused a stir. The only reason I was allowed to keep my gauge is because the TSA supervisor I had spoken to months earlier happened to be supervising that checkpoint and remembered our prior conversation. I was treated with courtesy and thankfully was not pressed for time, but the delay was not appreciated because I had taken steps to avoid problems. I mailed my gauges back to myself on the return trip. (I could have put the gauges in checked baggage but wanted to avoid that because I had to connect on different airlines.) Point being, if you need to fly with your weld gauges, put them in checked baggage because not all TSA employees interpret the rules the same way.

Chet Guilford