hello and welcome to the GMB

Regret I can't tell you where to buy more - am assuming this is a fairly modern drinking glass. You don't comment on the origin of this one - so presumably no help in that direction.
Generally, glasses made before the C20 have distinctive features that can tell much as to their age and origin etc., but more recent examples often lack these signs and your glass may be from almost anywhere, and assuming it is modern, then it's not possible usually to comment specifically on its provenance...........having said that, it's always possible that your glass may be from a known C20 designer, and might be recognisable in some way. Perhaps someone here might recognize the glass.
With older glasses, often the single most telling feature can be the foot - what does the underside look like - does it have a snapped pontil scar - or a ground/polished pontil depression - or, as is likely with this one, perhaps neither of these, just a smooth machine made finish.
You should find definitions of these terms on the GMB search facility - and you can read a lot there about drinking glasses.
Always very useful to also give the height of the glass, and to comment as to whether this is of lead glass or not (i.e. does it ring well if flicked).
Sorry to seem unhelpful - however, if I'm wrong and your glass has age, then perhaps you might provide additional description, and we can then offer possible suggestions - you might have reasons for believing this example to have some age.
Obviously, pix the right way up are preferred, and you might care to give details of the origin of this one.