Tasco made a one-piece base for Weaver-style rings. The same base was sold by Tasco with different model/part numbers, sometimes just the base and sometimes the base and rings as a set. They regularly show up on eBay. I bought a couple sets of them within the the last six months. Tasco referred to the one-piece bases as "bridge mounts" and some of the bases use 'BM' as a prefix for the model number.
The last time that I looked, GPC was selling a new swing-over rings and base combo for the Savage 99 for around $25.
I dug around in my boxes of bases and found two different Tasco bases for the Savage 99. What I think are the earlier/older style is the BM99 and the newer ring and base set is the 841L. I assume that the base was sold by itself at the 841, but I don't have one, so can't say for sure.
These Tasco bases sit a little lower than the 2-piece Weavers, so if you decide to buy and use them, you may need/want to buy a higher set of rings than you would use with the Weavers.
