If you buy from a place online, or over the phone out of state, you'll need to have the gun sent to an FFL (federal firearms licensed) dealer in your area, to do the transfer legally. I'd start by finding an FFL you like, for a price you like. Depending on where you are, and the amount of competition, they could charge anywhere from 20 to 100 bucks....since it's just filling in a form and calling for a background check, I'd look for the cheapest available.
If you buy from a store in state, no additional FFL required...so don't go for $20 savings online...cause after shipping you'll come out behind (with a $20 FFL fee).
Basically, any gun store worth their salt will probably offer to order any gun you can find online....just get a definitive price before committing.
A semi-auto .243 probably won't be in stock at most stores, so call the stores in your area first. In a two minute conversation you can find out if they have it, if they can get it, how much they charge for it, or how much they'd charge for a transfer (FFL) if you find it elsewhere.
If you tell us where you are, there might be a member nearby who can make a good suggestion on stores or an FFL.