You really can't. They haven't been manufactured for decades. The only way to do it is to find one from a private party (Ebay, gunbroker, etc) and even then, you're probably looking at around $50 for that little tiny piece of flimsy steel. Other options include using a bolt from a model 700 or having a gunsmith install a Sako extractor (involves drilling and milling your bolt). This is a permanent fix and costs around $100 total. It's a much better design.
The model 721 was produced from 1948 to 1962
The Remington Model 721 in .300 H&H was manufactured in 1948 with a 24" or 26" barrel.
Best left to a gunsmith
General information on the 721 can be found on the Remington website here: http://www.remington.com/library/history/firearm_models/centerfire/model_721.asp
Your Remington model 721 was produced from 1948-1962. If you can supply the 2,or 3 letter code on the barrel we can pinpoint the month and year of production.
100-300 USD
around 500$
100-400 or so
1 in 10
100-300 USD
Best left to a good gunsmith
The made them from 48 - 62, they are date code stamped on the barrel, the location and chart is on the Remington Society of America.