This is a difficult challenge.
To find cheap triathlon shorts in a small size.
In the UK you have the advantage the children's clothes are Value Added Tax free and can slip into children's clothes to your advantage.
Cheapest and best offer:
http://www.ebay.co.uk/itm/Speedo-Tri...em5892a c12bd
I know Bourne Sports, have used them before for running gear and know them to be entirely trustworthy. This looks like they are dumping unsellable stock. Bid quickly.
£6+£12 postage but they don't mail to USA or Canada, private message me if you need them sent on.
2nd offer:
http://www.amazon.co.uk/More-Mile-Tr...on+shorts+mens
Chinese made and branded, £15 don't know what US postage would be.
After these 2 shorts things get expensive.
Unless you fancy these:
http://www.ebay.co.uk/itm/BNWT-SPEED...32b083c0f#shId
They even mail to USA for £4