There are plenty of options for finding pre-loved spars. Many already mentioned, & even ebay.
As to figuring out if a perspective spar will work for your boat, if it's not obvious that it's strong enough. Then this book help you with 90% of the math which you need in order to figure it out Brion Toss Yacht Riggers, Sailboat Rigging::Search Results
And, FYI, a new edition of the book is coming out soon, as per a posting
But the math isn't very complicated. If you made it through Junior High, you can follow the calculations as ladi out in said reference, just fine.
Plus, it's got a Lot of other useful rigging
info. Much of it which will save you $ in the long run, & some of it's just plain fun.