VPI MOTOR PULLEY ??

I once had to go machine shop(one next to me is inside Napa autoparts) to cut me one for Sansui turntable that only had pulley for 50Hz by calculating diameter.
When you see turntable motor pulley, you will normally note barrel-like shape where the belt snaps. This preferably should stay same as in original one.
I was able to take measurements at different points and figure out the slope radius and mention on the draft.

pmotz,
it's not biggie, but you gotta be very good with numbers and math.
all you need is the brass rod, good mirror scale caliper, micrometer and calculator(optional) and cut at least a few pulleys with various curve radius and barrel size +- few micrometers to ensure proper speed.
if going automotive machine shop, request also to manufacture few sizes with provided engineering draft.
 

Purchasing lathe (gotta be kiddin' right?) that can cut with proper precision pulleys will worth a descent high-performance car such as Bentley Continental Azure Coupe(yea somewhere in few hundred k), but going just to trivial automotive machine shop with proper engineering drafts will solve your problem for a lot cheaper. 
Brass by far isn't best material. The best one would be rhodium.

Barrel shape of pulley is optional, but important at the same time.
Belt can travel over the straight cut pulleys and stays still over the barrel shape pulleys.

For the round-cut belt the pulley should have radial cut of slightly larger by .015mm to ensure easy fit without much traveling up-down. Please note that if radial cut radius is smaller than belt, the belt will wrap larger radius of pulley resulting higher platter rotation.

For my Sansui FR 1080 I've ordered 3 pulleys. one larger by few mcm, one exact and one smaller. I did not change the radius of the barrel. I only paid $19.xx for such endeavor.

My math had worked for me the following way:

1. Deriving the radius of a pulley.
Platter pulley with diameter D must rotate at 33.333... rpm and 45rpm
Motor pulley with diameter d must rotate at (as in your case) 300rpm
and ratio: 

D*s = d*S

where D is your large diameter of platter and s is your platter rpm and 
d is your motor diameter with S is motor RPM. (4th grade math?)

2. Deriving a radius of the barrel shape pulley.

Measure smallest dimeter d, height of any available turntable barrel shape pulley H and largest diameter D; divide both diameters by 2 to get radiuses r and R respectively; draw horizontal axis on your sheet of paper and plot 3 actual size vertical axis, originating from any arbitrary point of the horizontal axis -- 2 of size r and 1 of size R with R sized axis staying EXACTLY in the middle of 2 r sized axis giving you exactly 3 plotting points. As you know geometrically you can draw a circle via any 3 arbitrary points and that's how you define radius.
Graphically plot another horizontal axis via tops of r-sized vertical axis; continue to plot the straight lines of any reasonable size on both sides. Where straight line and curved line meet, plot a tangent lines going towards center till they meet and... BINGO you've found the barrel curve radius!

You can also refer  to http://math.stackexchange.com/questions/213658/get-the-equation-of-a-circle-when-given-3-points
if you prefer to solve series of quadratic equations instead.

Hi Bob,
Are you only mail-order dealer or have a store or warehouse in NC?

I share the same thoughts about location bob,
I was born in climate similar to Green Bay WI.

Bob,
Your area code though looked very close to where I dwell in North Carolina...