If you view the original listing page, sometimes there is a link at the top (look for a blue i banner) with a link to "view the item you purchased" or somesuch (usually on BIN items if the seller has revised the listing after you purchased).

If that doesn't work, try this, it should work as I just tried it on a purchase where I didn't have the "view item you purchased" link:

View the order details. In the URL will be two numbers, a transaction ID and the item number. Copy those.

Then, use this URL and modify it by putting the appropriate number in the appropriate spot - no spaces etc:

http://cgi.ebay.com.au/ws/eBayISAPI.dll?ViewItemVersion&item=[put the item number here]&view=all&tid=[put the trasaction ID here]