You probably won't spend the least amount of money by buying from a local shop, but realize the value of the service you receive and you may find you get the better deal by spending more money. There are many cottage guys making packs that you will not find at REI, but without much experience you're taking a real gamble that it has any chance of fitting you. Some of the online places offer a good return policy, but if you don't know what you want it could be a long (and possibly expensive with shipping costs) processes.

In terms of finding a knowledgeable salesperson, that is a risk, but I would try to visit a couple different shops if you can. REI will probably have the biggest selection. Try to go during the day when they are not as busy. You are more likely to be matched to the best salesperson available.