If I buy a field n forest dome tent, do I need to seam seal it?

Unfortunately nowhere. You will have have one made at a sailmakers using the old one as a pattern, or buy a new tent, or a generic tent flysheet.

https://www.google.com/search?q=replacement+rain+fly+for+Wenger+Appenzell+3+Room+Cabin+Dome+tent

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Contact the manufacturer/distributor to inquire.

try manufacturer for part replacement or thier customer service for assistance in part replacement

Call Mills Fleet Farm, it seems to be a house brand of theirs.
Customer service:

• Call us at 1-877-633-7456

This is a store brand from Mills Fleet and Farm. Call 1-877-633-7456 or go to http://www.fleetfarm.com/customer_service/?selectedSubCategory=customer_service

Best thing to do would be bring two tarps and suspened one above your tent and one underneaththe tent . . Also I would recommend seam sealing your tent. Also use a spray waterproofing on the outside and the inside just to be safe.

Actually Walmart have a tent that sleeps up to 12 (although I wouldn't be confident about fitting 12 adults in it), and it has windows. http://www.walmart.com/ip/Ozark-Trail-16-x-16-Cabin-Dome-Tent-Sleeps-12/20531614

If you sealed the seams properly, your tent should now be waterproof enough to keep the rain and dew out. That means it is also waterproof enough to keep moisture condensation in. As you sleep your body gives off moisture through perspiration and breathing. It's no so fun waking up wet. Most tents provide a roof vent and doors or windows with screens and I also advise you to get a ventilator to your tent in order to make your experience more comfortable.

Here's what I'd do:
Always use a ground cloth before putting the tent up.
Buy seam sealer, and recoat all of the seams.
Scotchgard by 3M will make the tent and fly more water repellent. Don't be stingy with the application.
Dry the tent if wet before storage to prevent the formation of mold and mildew. It isn't always practical to do this while moving your campsite, but do it as soon thereafter as practical.
Brush all debris out of and off of the tent before stowing. Clean off any stains and stubborn dirt with a mild detergent and water, flush well and dry before stowing.
Lubricate all zippers with wax.
Pack in extra tent pegs and cord. You never know when you'll want more strength.
OK, Does this stuff work? Yeppers! I have a Eureka! dome tent that I have used for 20 years, and it looks pretty close to brand new!
Best regards, from one happy camper to another!
--W/D--
Please feel free to rate this solution - thanx!