I have a Model 41V50 Rheem water heater (50 gal. natural gas) and needs to be replaced. However, I can't find this model at any of the local stores. Has this been discontinued? If so, what can I get for a replacement?

Call Rheem, they do not support gas change on all models:
http://waterheatertimer.org/Troubleshoot-Rheem-Tankless-water-heater.html

http://waterheatertimer.org/How-to-troubleshoot-gas-water-heater.html

Gene

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replace the entire water heater

No.
Replace tank.
Buy 40 gallon from local Lowes and install yourself.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

Under thin metal shell is a welded steel tank, surrounded by insulation and then covered with painted metal shell. The shell is what you look at.
The inside steel tank is made to withstand high pressure expansion that comes with heating water.
The steel tank has rusted out.
Turn gas control valve to vacation, so pilot light stays on, and then only heat enough water as needed until you can replace tank.

Prevent next tank from rusting by replacing anode rod, and ground water pipes.
http://waterheatertimer.org/How-to-maintain-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

21dv50 is discontinued. This is ordinary tank-type 50 gallon gas water heater.
Amazon sells 50 gallon 22V50F1 Rheem water heater for $420 with free shipping ... but who carries the warranty?

Frankly you can get in-store returns, and buy same quality gas water heater at local box store since Rheem makes GE water heaters at home Depot.
Other brands are equally good, as AO Smith makes Kenmore water heaters at Sears and Whirlpool at Lowes.

I prefer the AO Smith because maintenance is easier.
http://waterheatertimer.org/How-to-install-gas-water-heater.html

If you need further help, I’m available over the phone at https://www.6ya.com/expert/gene_9f0ef4df2f9897e7

You have 6-year 50 gallon gas water heater:
42V50 would be the newer model number. They make tall and short models.
http://www.rheem.com/product.aspx?id=CC643E03-4C15-4775-B139-E504A95651F7

All brands of water heater are good.
So any 50 gallon gas water heater from local box store or hardware is good product.
Tanks with longer warranty last longer. So 9-year and 12-year water heater will last longer, unless you have very hard water or odor in water and need to change frequently.
I recommend avoiding water heaters with electronic controls since they are not protected from power surge and can cost more to repair.

Additional resources:
http://waterheatertimer.org/How-to-install-gas-water-heater.html

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1) If tank is connected with single pipe to cold water line, then it could be expansion tank.
Add a comment with photo so I can see.
http://www.fixya.com/support/r6568559-post_photograph_fixya_questions_answers

2) Take photo to plumbing supply store.
Expansion tanks unscrew from pipe. Simply install it and it has internal pressure bladder.
Has to install on cold water line.

Add a comment with your experience so others with same-similar problem can benefit.

Also take advantage of fixya expert assistance live.
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Fixya is always less expensive than a service call.

The most common reason a pilot light will not stay lit is a bad thermocouple. How old is the water heater? If the pilot light will light, but then go out when you release the safety button, that is a sure sign the thermocouple is bad. You can buy a replacement thermocouple at your local home improvement store. There is a small chance that the control valve has gone bad, but that is highly unlikely.

If it is leaking from the tank, you'll have to replace the heater.

Yes, you will need a new heater. The inner tank has rusted thru and it is way cheeper to replace the whole water heater than trying to fix the old one. 

The model number you gave is for a 50 gallon tank, not 83 gallon.

50 gallon ones are readily available, including the same model..