I recently got Kamagra gel at the pharmacy on Thaniya which, I think, advertises 'purple drugs'. To find the shop, walk from Silom along Thaniya road towards Surawong and that pharmacy is on the left about three quarters of the way down Thaniya road, soon after a jewellery shop. I think it was 600bt for 7 items. Other pharmacies did not have Kamagra gel.