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 $100 total. It's a much better design.
The model 721 was produced from 1948 to 1962
The Remington Model 721 in .300 H&H was manufactured in 1948 with a 24" or 26" barrel.
Best left to a gunsmith
General information on the 721 can be found on the Remington website here: http://www.remington.com/library/history/firearm_models/centerfire/model_721.asp
Your Remington model 721 was produced from 1948-1962. If you can supply the 2,or 3 letter code on the barrel we can pinpoint the month and year of production.
around 500$
100-300 USD
100-400 or so
1 in 10
100-300 USD
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.
They are date stamped on the barrel, see related links for information on the Remington Society of America.