Gun show, gun shop, flea market. Legally the receiver IS the gun, and sales must be conducted as if it were a complete firearm.
There are on-line forums devoted to specific firearms, including the Mosin Nagant- they would be a good starting point. The link below will take you to one such forum.
They are $6.25 plus shipping from gunpartscorp.
Depends entirely on what type of shooting you like. Myself, I would buy the $80 M-N 91/30- and for $100, you could buy a multi-year supply of ammo.
Mosin-Nagant for around $60. 7.62*54r caliber. Nice reliable gun, i think 12 million were made.
Depends on which ammo you buy. I am shooting military surplus at about 15 cents per cartridge, but brass cased Remington will run $1 per cartridge.
If none of your local stores are carrying them, AIM Surplus and J&G sales probably have the best deals going on them right now. You may want to check Gun Broker and Auction Arms if you're looking for rarer/specific models, as the MN 91/30, and maybe some Finnish variants, such as the M38 or M39, will likely be the only models available at retailers.
Only the receiver requires a 4473 to be filled out and sent to the BATFE
you can buy them from importers for about $150 USD
For a Garmin Etrex GPS system, a receiver is not required. You can simply just buy a Garmin Etrex GPS and use it without having to buy any other accessories such as the receiver.
According to most reviews and ratings by customers and experts, a GPS receiver is a better buy than a GPS cradle. This is true not only for quality but for the price difference as well.
U-Haul
You can buy an HD DVR receiver straight from your cable provider. You can also find them at electronics stores. They are also available online at websites like amazon.com.