Ebay seems to have plenty of sellers offering High Density Memory Modules (RAM). Just a word of warning - the vast  majority (perhaps 75%) of motherboards do not support this format and it will either be identified in the BIOS as half the amount that the RAM stick is supposed to be or the system will not boot up at all. High Density DDR is most common. If you are unsure, ask the seller if it is high density.

The Seller is not obliged to make any refunds or similar in most cases as it is up to the buyer to make sure that components bought are compatible !!!

Only if you are 100% certain your motherboard/computer supports High-Density modules should they be purchased. Per Megabyte of RAM it is often cheaper.... but most of the time, cost and added hassle/inconvenience of an incompatibility issue is not worth the risk - so just stick to normal RAM.