West Marine is the HIGHEST retail mark up on hardware, never buy there unless its an emergency and you are just grabbing one or two in a pinch.
McMaster Carr is great for hard-to find and specialty fasteners. The website is great with detailed specs, drawings, and the best search and sort features of any website. I.e., you can sort by material so select stainless steel right off the top, then your screw size, thread pitch, etc and end up with exactly what you need. Only draw back on McMaster, a bit on the higher end of price, but I shop there frequently because of the above noted reasons.
http://www.allensfasteners.com/ This site is more geared toward automotive, but they have the best prices on ARP 12 Point Stainless Bolts, "I put that $hit on Everything!"
They also have excellent prices, fast shipping, and great assortment of bolts. When I needed 7/16 x 6 inch stainless to fix my outboard bracket, this is where I got em.
Fastenal is also pretty great, good website with excellent search criteria kind of like McMaster Carr, but they don't have the measured drawings. So far this only place I can fairly reasonable prices on Bronze hardware, also one of few places I could find external tooth lock washers made of Bronze! Prices are mostly fair to fairly good if you order large quantities.
Here is thing about the particular screws you are asking about, #8 @ 5 inches and #10 @ 4.5 inches... Not trying to be a jerk but that is just nuts. The reason they don't make such skinny machine screws to that length is that the fastener won't be able to handle the torsional stress over such a long shaft, even minor torque will break it.
You didn't mention pan head or flat-head, but they do have #10's in flat head only @ 4, 5, and 6 inches. #8's I cannot find over 4 inches of any type.
You also don't mention your application. Sometimes you need to modify your plan or fastener schedule... If it's possible to over bore you should up-grade to 1/4" or possibly even 5/16". Also, as others mentioned you might be far better off using threaded rod with cap nuts, would be cheaper that way too.