So, this is only to explain how to do so in the safest way. This is not to encourage people to do drugs, buying drugs is illegal. First, find a market. There are many lists of markets with reviews. I think the news site deepdotweb (dot) com maintains a good list. Then find a quality vendor. Most markets will keep track of a vendors number of sales and their reviews. The reviews should be 95%+ positive and you should read the bad reviews. In the case of LSD I'd suggest finding a vendor that sells just LSD. These are typically people who are really into LSD not just drug dealers and your interactions with them will tend to be better because they're often (not always) ideologically motivated rather than financially motivated, as such they often have better prices and service. Your prices should fall between $2/tab and $4.5/tab depending on how much you buy.

Warning: Do lookup how illegal possessing LSD is in your area. Often times the laws are somewhat insane (25 years for 1 gram of material including the weight of the tab). While it is very unlikely that anything would happen to you while buying online (it's kinda hard for the post office to detect 100 micrograms embedded in a piece of paper in a paper envelope) and LSD is perhaps one of the safest drugs to buy online. Make yourself aware of the associated risks before doing anything.