Fangraphs has a list of wOBA constants. They seem pretty arbitrary to me, though. How were those created?
From the Fangraphs wOBA explanatory page:
Exactly how much to weigh each of the components of wOBA was determined using linear weights.
Linear weights are calculated using the average Run Expectancy of each of those elements for that year, then scaled twice: once to scale so that an out is a 0 (instead of negative), and once to scale so that wOBA is on the same scale as league OBP minus IBBs.
Relevant sections of that page for the scaling:
For wOBA, we have the runs above average for walks (0.29), HBP (0.31), singles (0.44), doubles (0.74), triples (1.01), and home runs (1.39), but what we want to do now is put wOBA on a scale that will look like OBP in order to make it easier to understand. In OBP, an out is worth zero, so the first thing we want to do is adjust the run value scale so that an out is equal to zero.
But remember that we want wOBA to look like OBP. So we need to scale the entire thing so that the league’s wOBA is .313 (to match OBP with IBB removed). To do that, we divide .313/.250 and get 1.251, which we call the wOBA Scale.