AIM, Classic, Southern Ohio Gun, JG Sales, Buds Gun Sales
Research any supplier before ordering to see which ones people may or may not have had trouble with.
So far I have nothing but nice things to say about Southern Ohio Guns and Classicfirearms.com. Both got my info into the system immediately and have a pretty good selection even in this tough time.
Welcome to the highly addicting club! I would recommend that you first get signed copies of your C&R on file at a number of the vendors that Melnic helpfully listed above. I have found that AIM Surplus is a great vendor BUT it is a crap shoot with what you get. You can pay for "hand select" but in my experience it hasn't been worth the few extra bucks.
If you want to know exactly the gun you are getting (which is more expensive because these guys expend time and energy to find good inventory), try guys like Dennis Kroh at Empire Arms or Allan's Armory. There are lots of great local places including auctions where you can discover some true gems. I first waded into the Mosin Nagant area because of the pricing and wide assortment and variations available.
The key is to get on a few of the mailing lists and sign up for alerts. AIM Surplus will literally sell out of hundreds of surplus rifles in HOURS now.
Just an FYI; DO NOT SIGN the original. Make copies and sign them before you scan or mail out. If you Snail Mail a copy most vendors want to see it signed in ink, same if you hand a copy to an FFL at a Gun Show, they want you to sign it in ink.
I made copies with my DL under it so they both show up on the copy, most vendors want to see both.